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This is libitm.info, produced by makeinfo version 6.5 from libitm.texi.
Copyright (C) 2011-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license is included in the section entitled "GNU Free
Documentation License".
INFO-DIR-SECTION GNU Libraries
START-INFO-DIR-ENTRY
* libitm: (libitm). GNU Transactional Memory Library
END-INFO-DIR-ENTRY
This manual documents the GNU Transactional Memory Library.
Copyright (C) 2011-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license is included in the section entitled "GNU Free
Documentation License".

File: libitm.info, Node: Top, Next: Enabling libitm, Up: (dir)
Introduction
************
This manual documents the usage and internals of libitm, the GNU
Transactional Memory Library. It provides transaction support for
accesses to a process' memory, enabling easy-to-use synchronization of
accesses to shared memory by several threads.
* Menu:
* Enabling libitm:: How to enable libitm for your applications.
* C/C++ Language Constructs for TM::
Notes on the language-level interface supported
by gcc.
* The libitm ABI:: Notes on the external ABI provided by libitm.
* Internals:: Notes on libitm's internal synchronization.
* GNU Free Documentation License::
How you can copy and share this manual.
* Library Index:: Index of this documentation.

File: libitm.info, Node: Enabling libitm, Next: C/C++ Language Constructs for TM, Prev: Top, Up: Top
1 Enabling libitm
*****************
To activate support for TM in C/C++, the compile-time flag '-fgnu-tm'
must be specified. This enables TM language-level constructs such as
transaction statements (e.g., '__transaction_atomic', *note C/C++
Language Constructs for TM:: for details).

File: libitm.info, Node: C/C++ Language Constructs for TM, Next: The libitm ABI, Prev: Enabling libitm, Up: Top
2 C/C++ Language Constructs for TM
**********************************
Transactions are supported in C++ and C in the form of transaction
statements, transaction expressions, and function transactions. In the
following example, both 'a' and 'b' will be read and the difference will
be written to 'c', all atomically and isolated from other transactions:
__transaction_atomic { c = a - b; }
Therefore, another thread can use the following code to concurrently
update 'b' without ever causing 'c' to hold a negative value (and
without having to use other synchronization constructs such as locks or
C++11 atomics):
__transaction_atomic { if (a > b) b++; }
GCC follows the Draft Specification of Transactional Language
Constructs for C++ (v1.1)
(https://sites.google.com/site/tmforcplusplus/) in its implementation of
transactions.
The precise semantics of transactions are defined in terms of the
C++11/C11 memory model (see the specification). Roughly, transactions
provide synchronization guarantees that are similar to what would be
guaranteed when using a single global lock as a guard for all
transactions. Note that like other synchronization constructs in C/C++,
transactions rely on a data-race-free program (e.g., a nontransactional
write that is concurrent with a transactional read to the same memory
location is a data race).

File: libitm.info, Node: The libitm ABI, Next: Internals, Prev: C/C++ Language Constructs for TM, Up: Top
3 The libitm ABI
****************
The ABI provided by libitm is basically equal to the Linux variant of
Intel's current TM ABI specification document (Revision 1.1, May 6 2009)
but with the differences listed in this chapter. It would be good if
these changes would eventually be merged into a future version of this
specification. To ease look-up, the following subsections mirror the
structure of this specification.
3.1 [No changes] Objectives
===========================
3.2 [No changes] Non-objectives
===============================
3.3 Library design principles
=============================
3.3.1 [No changes] Calling conventions
--------------------------------------
3.3.2 [No changes] TM library algorithms
----------------------------------------
3.3.3 [No changes] Optimized load and store routines
----------------------------------------------------
3.3.4 [No changes] Aligned load and store routines
--------------------------------------------------
3.3.5 Data logging functions
----------------------------
The memory locations accessed with transactional loads and stores and
the memory locations whose values are logged must not overlap. This
required separation only extends to the scope of the execution of one
transaction including all the executions of all nested transactions.
The compiler must be consistent (within the scope of a single
transaction) about which memory locations are shared and which are not
shared with other threads (i.e., data must be accessed either
transactionally or nontransactionally). Otherwise, non-write-through TM
algorithms would not work.
For memory locations on the stack, this requirement extends to only
the lifetime of the stack frame that the memory location belongs to (or
the lifetime of the transaction, whichever is shorter). Thus, memory
that is reused for several stack frames could be target of both data
logging and transactional accesses; however, this is harmless because
these stack frames' lifetimes will end before the transaction finishes.
3.3.6 [No changes] Scatter/gather calls
---------------------------------------
3.3.7 [No changes] Serial and irrevocable mode
----------------------------------------------
3.3.8 [No changes] Transaction descriptor
-----------------------------------------
3.3.9 Store allocation
----------------------
There is no 'getTransaction' function.
3.3.10 [No changes] Naming conventions
--------------------------------------
3.3.11 Function pointer encryption
----------------------------------
Currently, this is not implemented.
3.4 Types and macros list
=========================
'_ITM_codeProperties' has changed, *note Starting a transaction:
txn-code-properties. '_ITM_srcLocation' is not used.
3.5 Function list
=================
3.5.1 Initialization and finalization functions
-----------------------------------------------
These functions are not part of the ABI.
3.5.2 [No changes] Version checking
-----------------------------------
3.5.3 [No changes] Error reporting
----------------------------------
3.5.4 [No changes] inTransaction call
-------------------------------------
3.5.5 State manipulation functions
----------------------------------
There is no 'getTransaction' function. Transaction identifiers for
nested transactions will be ordered but not necessarily sequential
(i.e., for a nested transaction's identifier IN and its enclosing
transaction's identifier IE, it is guaranteed that IN >= IE).
3.5.6 [No changes] Source locations
-----------------------------------
3.5.7 Starting a transaction
----------------------------
3.5.7.1 Transaction code properties
...................................
The bit 'hasNoXMMUpdate' is instead called 'hasNoVectorUpdate'. Iff it
is set, vector register save/restore is not necessary for any target
machine.
The 'hasNoFloatUpdate' bit ('0x0010') is new. Iff it is set,
floating point register save/restore is not necessary for any target
machine.
'undoLogCode' is not supported and a fatal runtime error will be
raised if this bit is set. It is not properly defined in the ABI why
barriers other than undo logging are not present; Are they not necessary
(e.g., a transaction operating purely on thread-local data) or have they
been omitted by the compiler because it thinks that some kind of global
synchronization (e.g., serial mode) might perform better? The
specification suggests that the latter might be the case, but the former
seems to be more useful.
The 'readOnly' bit ('0x4000') is new. *TODO* Lexical or dynamic
scope?
'hasNoRetry' is not supported. If this bit is not set, but
'hasNoAbort' is set, the library can assume that transaction rollback
will not be requested.
It would be useful if the absence of externally-triggered rollbacks
would be reported for the dynamic scope as well, not just for the
lexical scope ('hasNoAbort'). Without this, a library cannot exploit
this together with flat nesting.
'exceptionBlock' is not supported because exception blocks are not
used.
3.5.7.2 [No changes] Windows exception state
............................................
3.5.7.3 [No changes] Other machine state
........................................
3.5.7.4 [No changes] Results from beginTransaction
..................................................
3.5.8 Aborting a transaction
----------------------------
'_ITM_rollbackTransaction' is not supported. '_ITM_abortTransaction' is
supported but the abort reasons 'exceptionBlockAbort', 'TMConflict', and
'userRetry' are not supported. There are no exception blocks in
general, so the related cases also do not have to be considered. To
encode '__transaction_cancel [[outer]]', compilers must set the new
'outerAbort' bit ('0x10') additionally to the 'userAbort' bit in the
abort reason.
3.5.9 Committing a transaction
------------------------------
The exception handling (EH) scheme is different. The Intel ABI requires
the '_ITM_tryCommitTransaction' function that will return even when the
commit failed and will have to be matched with calls to either
'_ITM_abortTransaction' or '_ITM_commitTransaction'. In contrast, gcc
relies on transactional wrappers for the functions of the Exception
Handling ABI and on one additional commit function (shown below). This
allows the TM to keep track of EH internally and thus it does not have
to embed the cleanup of EH state into the existing EH code in the
program. '_ITM_tryCommitTransaction' is not supported.
'_ITM_commitTransactionToId' is also not supported because the
propagation of thrown exceptions will not bypass commits of nested
transactions.
void _ITM_commitTransactionEH(void *exc_ptr) ITM_REGPARM;
void *_ITM_cxa_allocate_exception (size_t);
void _ITM_cxa_free_exception (void *exc_ptr);
void _ITM_cxa_throw (void *obj, void *tinfo, void (*dest) (void *));
void *_ITM_cxa_begin_catch (void *exc_ptr);
void _ITM_cxa_end_catch (void);
The EH scheme changed in version 6 of GCC. Previously, the compiler
added a call to '_ITM_commitTransactionEH' to commit a transaction if an
exception could be in flight at this position in the code; 'exc_ptr' is
the address of the current exception and must be non-zero. Now, the
compiler must catch all exceptions that are about to be thrown out of a
transaction and call '_ITM_commitTransactionEH' from the catch clause,
with 'exc_ptr' being zero.
Note that the old EH scheme never worked completely in GCC's
implementation; libitm currently does not try to be compatible with the
old scheme.
The '_ITM_cxa...' functions are transactional wrappers for the
respective '__cxa...' functions and must be called instead of these in
transactional code. '_ITM_cxa_free_exception' is new in GCC 6.
To support this EH scheme, libstdc++ needs to provide one additional
function ('_cxa_tm_cleanup'), which is used by the TM to clean up the
exception handling state while rolling back a transaction:
void __cxa_tm_cleanup (void *unthrown_obj, void *cleanup_exc,
unsigned int caught_count);
Since GCC 6, 'unthrown_obj' is not used anymore and always null;
prior to that, 'unthrown_obj' is non-null if the program called
'__cxa_allocate_exception' for this exception but did not yet called
'__cxa_throw' for it. 'cleanup_exc' is non-null if the program is
currently processing a cleanup along an exception path but has not
caught this exception yet. 'caught_count' is the nesting depth of
'__cxa_begin_catch' within the transaction (which can be counted by the
TM using '_ITM_cxa_begin_catch' and '_ITM_cxa_end_catch');
'__cxa_tm_cleanup' then performs rollback by essentially performing
'__cxa_end_catch' that many times.
3.5.10 Exception handling support
---------------------------------
Currently, there is no support for functionality like
'__transaction_cancel throw' as described in the C++ TM specification.
Supporting this should be possible with the EH scheme explained
previously because via the transactional wrappers for the EH ABI, the TM
is able to observe and intercept EH.
3.5.11 [No changes] Transition to serial-irrevocable mode
---------------------------------------------------------
3.5.12 [No changes] Data transfer functions
-------------------------------------------
3.5.13 [No changes] Transactional memory copies
-----------------------------------------------
3.5.14 Transactional versions of memmove
----------------------------------------
If either the source or destination memory region is to be accessed
nontransactionally, then source and destination regions must not be
overlapping. The respective '_ITM_memmove' functions are still
available but a fatal runtime error will be raised if such regions do
overlap. To support this functionality, the ABI would have to specify
how the intersection of the regions has to be accessed (i.e.,
transactionally or nontransactionally).
3.5.15 [No changes] Transactional versions of memset
----------------------------------------------------
3.5.16 [No changes] Logging functions
-------------------------------------
3.5.17 User-registered commit and undo actions
----------------------------------------------
Commit actions will get executed in the same order in which the
respective calls to '_ITM_addUserCommitAction' happened. Only
'_ITM_noTransactionId' is allowed as value for the
'resumingTransactionId' argument. Commit actions get executed after
privatization safety has been ensured.
Undo actions will get executed in reverse order compared to the order
in which the respective calls to '_ITM_addUserUndoAction' happened. The
ordering of undo actions w.r.t. the roll-back of other actions (e.g.,
data transfers or memory allocations) is undefined.
'_ITM_getThreadnum' is not supported currently because its only
purpose is to provide a thread ID that matches some assumed performance
tuning output, but this output is not part of the ABI nor further
defined by it.
'_ITM_dropReferences' is not supported currently because its
semantics and the intention behind it is not entirely clear. The
specification suggests that this function is necessary because of
certain orderings of data transfer undos and the releasing of memory
regions (i.e., privatization). However, this ordering is never defined,
nor is the ordering of dropping references w.r.t. other events.
3.5.18 [New] Transactional indirect calls
-----------------------------------------
Indirect calls (i.e., calls through a function pointer) within
transactions should execute the transactional clone of the original
function (i.e., a clone of the original that has been fully instrumented
to use the TM runtime), if such a clone is available. The runtime
provides two functions to register/deregister clone tables:
struct clone_entry
{
void *orig, *clone;
};
void _ITM_registerTMCloneTable (clone_entry *table, size_t entries);
void _ITM_deregisterTMCloneTable (clone_entry *table);
Registered tables must be writable by the TM runtime, and must be
live throughout the life-time of the TM runtime.
*TODO* The intention was always to drop the registration functions
entirely, and create a new ELF Phdr describing the linker-sorted table.
Much like what currently happens for 'PT_GNU_EH_FRAME'. This work kept
getting bogged down in how to represent the N different code generation
variants. We clearly needed at least two--SW and HW transactional
clones--but there was always a suggestion of more variants for different
TM assumptions/invariants.
The compiler can then use two TM runtime functions to perform
indirect calls in transactions:
void *_ITM_getTMCloneOrIrrevocable (void *function) ITM_REGPARM;
void *_ITM_getTMCloneSafe (void *function) ITM_REGPARM;
If there is a registered clone for supplied function, both will
return a pointer to the clone. If not, the first runtime function will
attempt to switch to serial-irrevocable mode and return the original
pointer, whereas the second will raise a fatal runtime error.
3.5.19 [New] Transactional dynamic memory management
----------------------------------------------------
void *_ITM_malloc (size_t)
__attribute__((__malloc__)) ITM_PURE;
void *_ITM_calloc (size_t, size_t)
__attribute__((__malloc__)) ITM_PURE;
void _ITM_free (void *) ITM_PURE;
These functions are essentially transactional wrappers for 'malloc',
'calloc', and 'free'. Within transactions, the compiler should replace
calls to the original functions with calls to the wrapper functions.
libitm also provides transactional clones of C++ memory management
functions such as global operator new and delete. They are part of
libitm for historic reasons but do not need to be part of this ABI.
3.6 [No changes] Future Enhancements to the ABI
===============================================
3.7 Sample code
===============
The code examples might not be correct w.r.t. the current version of
the ABI, especially everything related to exception handling.
3.8 [New] Memory model
======================
The ABI should define a memory model and the ordering that is guaranteed
for data transfers and commit/undo actions, or at least refer to another
memory model that needs to be preserved. Without that, the compiler
cannot ensure the memory model specified on the level of the programming
language (e.g., by the C++ TM specification).
For example, if a transactional load is ordered before another
load/store, then the TM runtime must also ensure this ordering when
accessing shared state. If not, this might break the kind of
publication safety used in the C++ TM specification. Likewise, the TM
runtime must ensure privatization safety.

File: libitm.info, Node: Internals, Next: GNU Free Documentation License, Prev: The libitm ABI, Up: Top
4 Internals
***********
4.1 TM methods and method groups
================================
libitm supports several ways of synchronizing transactions with each
other. These TM methods (or TM algorithms) are implemented in the form
of subclasses of 'abi_dispatch', which provide methods for transactional
loads and stores as well as callbacks for rollback and commit. All
methods that are compatible with each other (i.e., that let concurrently
running transactions still synchronize correctly even if different
methods are used) belong to the same TM method group. Pointers to TM
methods can be obtained using the factory methods prefixed with
'dispatch_' in 'libitm_i.h'. There are two special methods,
'dispatch_serial' and 'dispatch_serialirr', that are compatible with all
methods because they run transactions completely in serial mode.
4.1.1 TM method life cycle
--------------------------
The state of TM methods does not change after construction, but they do
alter the state of transactions that use this method. However, because
per-transaction data gets used by several methods, 'gtm_thread' is
responsible for setting an initial state that is useful for all methods.
After that, methods are responsible for resetting/clearing this state on
each rollback or commit (of outermost transactions), so that the
transaction executed next is not affected by the previous transaction.
There is also global state associated with each method group, which
is initialized and shut down ('method_group::init()' and 'fini()') when
switching between method groups (see 'retry.cc').
4.1.2 Selecting the default method
----------------------------------
The default method that libitm uses for freshly started transactions
(but not necessarily for restarted transactions) can be set via an
environment variable ('ITM_DEFAULT_METHOD'), whose value should be equal
to the name of one of the factory methods returning abi_dispatch
subclasses but without the "dispatch_" prefix (e.g., "serialirr" instead
of 'GTM::dispatch_serialirr()').
Note that this environment variable is only a hint for libitm and
might not be supported in the future.
4.2 Nesting: flat vs. closed
============================
We support two different kinds of nesting of transactions. In the case
of _flat nesting_, the nesting structure is flattened and all nested
transactions are subsumed by the enclosing transaction. In contrast,
with _closed nesting_, nested transactions that have not yet committed
can be rolled back separately from the enclosing transactions; when they
commit, they are subsumed by the enclosing transaction, and their
effects will be finally committed when the outermost transaction
commits. _Open nesting_ (where nested transactions can commit
independently of the enclosing transactions) are not supported.
Flat nesting is the default nesting mode, but closed nesting is
supported and used when transactions contain user-controlled aborts
('__transaction_cancel' statements). We assume that user-controlled
aborts are rare in typical code and used mostly in exceptional
situations. Thus, it makes more sense to use flat nesting by default to
avoid the performance overhead of the additional checkpoints required
for closed nesting. User-controlled aborts will correctly abort the
innermost enclosing transaction, whereas the whole (i.e., outermost)
transaction will be restarted otherwise (e.g., when a transaction
encounters data conflicts during optimistic execution).
4.3 Locking conventions
=======================
This section documents the locking scheme and rules for all uses of
locking in libitm. We have to support serial(-irrevocable) mode, which
is implemented using a global lock as explained next (called the _serial
lock_). To simplify the overall design, we use the same lock as
catch-all locking mechanism for other infrequent tasks such as
(de)registering clone tables or threads. Besides the serial lock, there
are _per-method-group locks_ that are managed by specific method groups
(i.e., groups of similar TM concurrency control algorithms), and
lock-like constructs for quiescence-based operations such as ensuring
privatization safety.
Thus, the actions that participate in the libitm-internal locking are
either _active transactions_ that do not run in serial mode, _serial
transactions_ (which (are about to) run in serial mode), and management
tasks that do not execute within a transaction but have acquired the
serial mode like a serial transaction would do (e.g., to be able to
register threads with libitm). Transactions become active as soon as
they have successfully used the serial lock to announce this globally
(*note Serial lock implementation: serial-lock-impl.). Likewise,
transactions become serial transactions as soon as they have acquired
the exclusive rights provided by the serial lock (i.e., serial mode,
which also means that there are no other concurrent active or serial
transactions). Note that active transactions can become serial
transactions when they enter serial mode during the runtime of the
transaction.
4.3.1 State-to-lock mapping
---------------------------
Application data is protected by the serial lock if there is a serial
transaction and no concurrently running active transaction (i.e.,
non-serial). Otherwise, application data is protected by the currently
selected method group, which might use per-method-group locks or other
mechanisms. Also note that application data that is about to be
privatized might not be allowed to be accessed by nontransactional code
until privatization safety has been ensured; the details of this are
handled by the current method group.
libitm-internal state is either protected by the serial lock or
accessed through custom concurrent code. The latter applies to the
public/shared part of a transaction object and most typical
method-group-specific state.
The former category (protected by the serial lock) includes:
* The list of active threads that have used transactions.
* The tables that map functions to their transactional clones.
* The current selection of which method group to use.
* Some method-group-specific data, or invariants of this data. For
example, resetting a method group to its initial state is handled
by switching to the same method group, so the serial lock protects
such resetting as well.
In general, such state is immutable whenever there exists an active
(non-serial) transaction. If there is no active transaction, a serial
transaction (or a thread that is not currently executing a transaction
but has acquired the serial lock) is allowed to modify this state (but
must of course be careful to not surprise the current method group's
implementation with such modifications).
4.3.2 Lock acquisition order
----------------------------
To prevent deadlocks, locks acquisition must happen in a globally
agreed-upon order. Note that this applies to other forms of blocking
too, but does not necessarily apply to lock acquisitions that do not
block (e.g., trylock() calls that do not get retried forever). Note
that serial transactions are never return back to active transactions
until the transaction has committed. Likewise, active transactions stay
active until they have committed. Per-method-group locks are typically
also not released before commit.
Lock acquisition / blocking rules:
* Transactions must become active or serial before they are allowed
to use method-group-specific locks or blocking (i.e., the serial
lock must be acquired before those other locks, either in serial or
nonserial mode).
* Any number of threads that do not currently run active transactions
can block while trying to get the serial lock in exclusive mode.
Note that active transactions must not block when trying to upgrade
to serial mode unless there is no other transaction that is trying
that (the latter is ensured by the serial lock implementation.
* Method groups must prevent deadlocks on their locks. In
particular, they must also be prepared for another active
transaction that has acquired method-group-specific locks but is
blocked during an attempt to upgrade to being a serial transaction.
See below for details.
* Serial transactions can acquire method-group-specific locks because
there will be no other active nor serial transaction.
There is no single rule for per-method-group blocking because this
depends on when a TM method might acquire locks. If no active
transaction can upgrade to being a serial transaction after it has
acquired per-method-group locks (e.g., when those locks are only
acquired during an attempt to commit), then the TM method does not need
to consider a potential deadlock due to serial mode.
If there can be upgrades to serial mode after the acquisition of
per-method-group locks, then TM methods need to avoid those deadlocks:
* When upgrading to a serial transaction, after acquiring exclusive
rights to the serial lock but before waiting for concurrent active
transactions to finish (*note Serial lock implementation:
serial-lock-impl. for details), we have to wake up all active
transactions waiting on the upgrader's per-method-group locks.
* Active transactions blocking on per-method-group locks need to
check the serial lock and abort if there is a pending serial
transaction.
* Lost wake-ups have to be prevented (e.g., by changing a bit in each
per-method-group lock before doing the wake-up, and only blocking
on this lock using a futex if this bit is not group).
*TODO*: Can reuse serial lock for gl-*? And if we can, does it make
sense to introduce further complexity in the serial lock? For gl-*, we
can really only avoid an abort if we do -wb and -vbv.
4.3.3 Serial lock implementation
--------------------------------
The serial lock implementation is optimized towards assuming that serial
transactions are infrequent and not the common case. However, the
performance of entering serial mode can matter because when only few
transactions are run concurrently or if there are few threads, then it
can be efficient to run transactions serially.
The serial lock is similar to a multi-reader-single-writer lock in
that there can be several active transactions but only one serial
transaction. However, we do want to avoid contention (in the lock
implementation) between active transactions, so we split up the reader
side of the lock into per-transaction flags that are true iff the
transaction is active. The exclusive writer side remains a shared
single flag, which is acquired using a CAS, for example. On the
fast-path, the serial lock then works similar to Dekker's algorithm but
with several reader flags that a serial transaction would have to check.
A serial transaction thus requires a list of all threads with
potentially active transactions; we can use the serial lock itself to
protect this list (i.e., only threads that have acquired the serial lock
can modify this list).
We want starvation-freedom for the serial lock to allow for using it
to ensure progress for potentially starved transactions (*note Progress
Guarantees: progress-guarantees. for details). However, this is
currently not enforced by the implementation of the serial lock.
Here is pseudo-code for the read/write fast paths of acquiring the
serial lock (read-to-write upgrade is similar to write_lock:
// read_lock:
tx->shared_state |= active;
__sync_synchronize(); // or STLD membar, or C++0x seq-cst fence
while (!serial_lock.exclusive)
if (spinning_for_too_long) goto slowpath;
// write_lock:
if (CAS(&serial_lock.exclusive, 0, this) != 0)
goto slowpath; // writer-writer contention
// need a membar here, but CAS already has full membar semantics
bool need_blocking = false;
for (t: all txns)
{
for (;t->shared_state & active;)
if (spinning_for_too_long) { need_blocking = true; break; }
}
if (need_blocking) goto slowpath;
Releasing a lock in this spin-lock version then just consists of
resetting 'tx->shared_state' to inactive or clearing
'serial_lock.exclusive'.
However, we can't rely on a pure spinlock because we need to get the
OS involved at some time (e.g., when there are more threads than CPUs to
run on). Therefore, the real implementation falls back to a blocking
slow path, either based on pthread mutexes or Linux futexes.
4.3.4 Reentrancy
----------------
libitm has to consider the following cases of reentrancy:
* Transaction calls unsafe code that starts a new transaction: The
outer transaction will become a serial transaction before executing
unsafe code. Therefore, nesting within serial transactions must
work, even if the nested transaction is called from within
uninstrumented code.
* Transaction calls either a transactional wrapper or safe code,
which in turn starts a new transaction: It is not yet defined in
the specification whether this is allowed. Thus, it is undefined
whether libitm supports this.
* Code that starts new transactions might be called from within any
part of libitm: This kind of reentrancy would likely be rather
complex and can probably be avoided. Therefore, it is not
supported.
4.3.5 Privatization safety
--------------------------
Privatization safety is ensured by libitm using a quiescence-based
approach. Basically, a privatizing transaction waits until all
concurrent active transactions will either have finished (are not active
anymore) or operate on a sufficiently recent snapshot to not access the
privatized data anymore. This happens after the privatizing transaction
has stopped being an active transaction, so waiting for quiescence does
not contribute to deadlocks.
In method groups that need to ensure publication safety explicitly,
active transactions maintain a flag or timestamp in the public/shared
part of the transaction descriptor. Before blocking, privatizers need
to let the other transactions know that they should wake up the
privatizer.
*TODO* Ho to implement the waiters? Should those flags be
per-transaction or at a central place? We want to avoid one wake/wait
call per active transactions, so we might want to use either a tree or
combining to reduce the syscall overhead, or rather spin for a long
amount of time instead of doing blocking. Also, it would be good if
only the last transaction that the privatizer waits for would do the
wake-up.
4.3.6 Progress guarantees
-------------------------
Transactions that do not make progress when using the current TM method
will eventually try to execute in serial mode. Thus, the serial lock's
progress guarantees determine the progress guarantees of the whole TM.
Obviously, we at least need deadlock-freedom for the serial lock, but it
would also be good to provide starvation-freedom (informally, all
threads will finish executing a transaction eventually iff they get
enough cycles).
However, the scheduling of transactions (e.g., thread scheduling by
the OS) also affects the handling of progress guarantees by the TM.
First, the TM can only guarantee deadlock-freedom if threads do not get
stopped. Likewise, low-priority threads can starve if they do not get
scheduled when other high-priority threads get those cycles instead.
If all threads get scheduled eventually, correct lock implementations
will provide deadlock-freedom, but might not provide starvation-freedom.
We can either enforce the latter in the TM's lock implementation, or
assume that the scheduling is sufficiently random to yield a
probabilistic guarantee that no thread will starve (because eventually,
a transaction will encounter a scheduling that will allow it to run).
This can indeed work well in practice but is not necessarily guaranteed
to work (e.g., simple spin locks can be pretty efficient).
Because enforcing stronger progress guarantees in the TM has a higher
runtime overhead, we focus on deadlock-freedom right now and assume that
the threads will get scheduled eventually by the OS (but don't consider
threads with different priorities). We should support
starvation-freedom for serial transactions in the future. Everything
beyond that is highly related to proper contention management across all
of the TM (including with TM method to choose), and is future work.
*TODO* Handling thread priorities: We want to avoid priority
inversion but it's unclear how often that actually matters in practice.
Workloads that have threads with different priorities will likely also
require lower latency or higher throughput for high-priority threads.
Therefore, it probably makes not that much sense (except for eventual
progress guarantees) to use priority inheritance until the TM has
priority-aware contention management.

File: libitm.info, Node: GNU Free Documentation License, Next: Library Index, Prev: Internals, Up: Top
GNU Free Documentation License
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Tag Table:
Node: Top1141
Node: Enabling libitm2045
Node: C/C++ Language Constructs for TM2440
Node: The libitm ABI3923
Ref: txn-code-properties7721
Node: Internals18814
Ref: serial-lock-impl28852
Ref: progress-guarantees33613
Node: GNU Free Documentation License35891
Node: Library Index61021

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