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/* elide.h: Generic lock elision support.
Copyright (C) 2014-2022 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef ELIDE_H
#define ELIDE_H 1
#include <hle.h>
#include <elision-conf.h>
#include <atomic.h>
/* Adapt elision with ADAPT_COUNT and STATUS and decide retries. */
static inline bool
elision_adapt(signed char *adapt_count, unsigned int status)
{
if (status & _XABORT_RETRY)
return false;
if ((status & _XABORT_EXPLICIT)
&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
{
/* Right now we skip here. Better would be to wait a bit
and retry. This likely needs some spinning. Be careful
to avoid writing the lock.
Using relaxed MO and separate atomic accesses is sufficient because
adapt_count is just a hint. */
if (atomic_load_relaxed (adapt_count) != __elision_aconf.skip_lock_busy)
atomic_store_relaxed (adapt_count, __elision_aconf.skip_lock_busy);
}
/* Internal abort. There is no chance for retry.
Use the normal locking and next time use lock.
Be careful to avoid writing to the lock. See above for MO. */
else if (atomic_load_relaxed (adapt_count)
!= __elision_aconf.skip_lock_internal_abort)
atomic_store_relaxed (adapt_count,
__elision_aconf.skip_lock_internal_abort);
return true;
}
/* is_lock_free must be executed inside the transaction */
/* Returns true if lock defined by IS_LOCK_FREE was elided.
ADAPT_COUNT is a per-lock state variable; it must be accessed atomically
to avoid data races but is just a hint, so using relaxed MO and separate
atomic loads and stores instead of atomic read-modify-write operations is
sufficient. */
#define ELIDE_LOCK(adapt_count, is_lock_free) \
({ \
int ret = 0; \
\
if (atomic_load_relaxed (&(adapt_count)) <= 0) \
{ \
for (int i = __elision_aconf.retry_try_xbegin; i > 0; i--) \
{ \
unsigned int status; \
if ((status = _xbegin ()) == _XBEGIN_STARTED) \
{ \
if (is_lock_free) \
{ \
ret = 1; \
break; \
} \
_xabort (_ABORT_LOCK_BUSY); \
} \
if (!elision_adapt (&(adapt_count), status)) \
break; \
} \
} \
else \
atomic_store_relaxed (&(adapt_count), \
atomic_load_relaxed (&(adapt_count)) - 1); \
ret; \
})
/* Returns true if lock defined by IS_LOCK_FREE was try-elided.
ADAPT_COUNT is a per-lock state variable. */
#define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) ({ \
int ret = 0; \
if (__elision_aconf.retry_try_xbegin > 0) \
{ \
if (write) \
_xabort (_ABORT_NESTED_TRYLOCK); \
ret = ELIDE_LOCK (adapt_count, is_lock_free); \
} \
ret; \
})
/* Returns true if lock defined by IS_LOCK_FREE was elided. The call
to _xend crashes if the application incorrectly tries to unlock a
lock which has not been locked. */
#define ELIDE_UNLOCK(is_lock_free) \
({ \
int ret = 0; \
if (is_lock_free) \
{ \
_xend (); \
ret = 1; \
} \
ret; \
})
#endif
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