diff options
Diffstat (limited to 'sysdeps/x86/elide.h')
| -rw-r--r-- | sysdeps/x86/elide.h | 32 |
1 files changed, 20 insertions, 12 deletions
diff --git a/sysdeps/x86/elide.h b/sysdeps/x86/elide.h index 8691e6673d..8d5589902f 100644 --- a/sysdeps/x86/elide.h +++ b/sysdeps/x86/elide.h @@ -1,5 +1,5 @@ /* elide.h: Generic lock elision support. - Copyright (C) 2014-2016 Free Software Foundation, Inc. + Copyright (C) 2014-2018 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 @@ -20,8 +20,8 @@ #include <hle.h> #include <elision-conf.h> +#include <atomic.h> -#define ACCESS_ONCE(x) (* (volatile typeof(x) *) &(x)) /* Adapt elision with ADAPT_COUNT and STATUS and decide retries. */ @@ -35,28 +35,35 @@ elision_adapt(signed char *adapt_count, unsigned int status) { /* 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. */ - if (*adapt_count != __elision_aconf.skip_lock_busy) - ACCESS_ONCE (*adapt_count) = __elision_aconf.skip_lock_busy; + 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. */ - else if (*adapt_count != __elision_aconf.skip_lock_internal_abort) - ACCESS_ONCE (*adapt_count) = __elision_aconf.skip_lock_internal_abort; + 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 pointer to per-lock state variable. */ + 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 ((adapt_count) <= 0) \ + if (atomic_load_relaxed (&(adapt_count)) <= 0) \ { \ for (int i = __elision_aconf.retry_try_xbegin; i > 0; i--) \ { \ @@ -75,12 +82,13 @@ elision_adapt(signed char *adapt_count, unsigned int status) } \ } \ else \ - (adapt_count)--; /* missing updates ok */ \ + 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 pointer to per-lock state variable. */ + ADAPT_COUNT is a per-lock state variable. */ #define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) ({ \ int ret = 0; \ |