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120 lines
3.9 KiB
C
120 lines
3.9 KiB
C
/* elide.h: Generic lock elision support.
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Copyright (C) 2014-2024 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef ELIDE_H
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#define ELIDE_H 1
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#include <hle.h>
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#include <elision-conf.h>
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#include <atomic.h>
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/* Adapt elision with ADAPT_COUNT and STATUS and decide retries. */
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static inline bool
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elision_adapt(signed char *adapt_count, unsigned int status)
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{
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if (status & _XABORT_RETRY)
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return false;
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if ((status & _XABORT_EXPLICIT)
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&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
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{
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/* Right now we skip here. Better would be to wait a bit
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and retry. This likely needs some spinning. Be careful
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to avoid writing the lock.
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Using relaxed MO and separate atomic accesses is sufficient because
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adapt_count is just a hint. */
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if (atomic_load_relaxed (adapt_count) != __elision_aconf.skip_lock_busy)
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atomic_store_relaxed (adapt_count, __elision_aconf.skip_lock_busy);
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}
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/* Internal abort. There is no chance for retry.
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Use the normal locking and next time use lock.
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Be careful to avoid writing to the lock. See above for MO. */
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else if (atomic_load_relaxed (adapt_count)
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!= __elision_aconf.skip_lock_internal_abort)
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atomic_store_relaxed (adapt_count,
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__elision_aconf.skip_lock_internal_abort);
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return true;
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}
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/* is_lock_free must be executed inside the transaction */
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/* Returns true if lock defined by IS_LOCK_FREE was elided.
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ADAPT_COUNT is a per-lock state variable; it must be accessed atomically
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to avoid data races but is just a hint, so using relaxed MO and separate
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atomic loads and stores instead of atomic read-modify-write operations is
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sufficient. */
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#define ELIDE_LOCK(adapt_count, is_lock_free) \
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({ \
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int ret = 0; \
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\
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if (atomic_load_relaxed (&(adapt_count)) <= 0) \
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{ \
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for (int i = __elision_aconf.retry_try_xbegin; i > 0; i--) \
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{ \
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unsigned int status; \
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if ((status = _xbegin ()) == _XBEGIN_STARTED) \
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{ \
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if (is_lock_free) \
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{ \
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ret = 1; \
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break; \
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} \
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_xabort (_ABORT_LOCK_BUSY); \
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} \
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if (!elision_adapt (&(adapt_count), status)) \
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break; \
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} \
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} \
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else \
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atomic_store_relaxed (&(adapt_count), \
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atomic_load_relaxed (&(adapt_count)) - 1); \
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ret; \
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})
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/* Returns true if lock defined by IS_LOCK_FREE was try-elided.
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ADAPT_COUNT is a per-lock state variable. */
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#define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) ({ \
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int ret = 0; \
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if (__elision_aconf.retry_try_xbegin > 0) \
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{ \
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if (write) \
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_xabort (_ABORT_NESTED_TRYLOCK); \
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ret = ELIDE_LOCK (adapt_count, is_lock_free); \
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} \
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ret; \
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})
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/* Returns true if lock defined by IS_LOCK_FREE was elided. The call
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to _xend crashes if the application incorrectly tries to unlock a
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lock which has not been locked. */
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#define ELIDE_UNLOCK(is_lock_free) \
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({ \
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int ret = 0; \
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if (is_lock_free) \
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{ \
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_xend (); \
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ret = 1; \
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} \
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ret; \
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})
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#endif
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