diff options
Diffstat (limited to 'sysdeps')
| -rw-r--r-- | sysdeps/unix/sysv/linux/x86/elision-lock.c | 28 | ||||
| -rw-r--r-- | sysdeps/unix/sysv/linux/x86/elision-trylock.c | 18 | ||||
| -rw-r--r-- | sysdeps/x86/elide.h | 30 |
3 files changed, 50 insertions, 26 deletions
diff --git a/sysdeps/unix/sysv/linux/x86/elision-lock.c b/sysdeps/unix/sysv/linux/x86/elision-lock.c index 5e66960123..c05ade4722 100644 --- a/sysdeps/unix/sysv/linux/x86/elision-lock.c +++ b/sysdeps/unix/sysv/linux/x86/elision-lock.c @@ -44,7 +44,13 @@ int __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) { - if (*adapt_count <= 0) + /* adapt_count can be accessed concurrently; these accesses can be both + inside of transactions (if critical sections are nested and the outer + critical section uses lock elision) and outside of transactions. Thus, + we need to use atomic accesses to avoid data races. However, the + value of adapt_count is just a hint, so relaxed MO accesses are + sufficient. */ + if (atomic_load_relaxed (adapt_count) <= 0) { unsigned status; int try_xbegin; @@ -70,15 +76,20 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) && _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. */ - if (*adapt_count != aconf.skip_lock_busy) - *adapt_count = aconf.skip_lock_busy; + and retry. This likely needs some spinning. See + above for why relaxed MO is sufficient. */ + if (atomic_load_relaxed (adapt_count) + != aconf.skip_lock_busy) + atomic_store_relaxed (adapt_count, 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 != aconf.skip_lock_internal_abort) - *adapt_count = aconf.skip_lock_internal_abort; + Be careful to avoid writing to the lock. See above for why + relaxed MO is sufficient. */ + else if (atomic_load_relaxed (adapt_count) + != aconf.skip_lock_internal_abort) + atomic_store_relaxed (adapt_count, + aconf.skip_lock_internal_abort); break; } } @@ -87,7 +98,8 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) { /* Use a normal lock until the threshold counter runs out. Lost updates possible. */ - (*adapt_count)--; + atomic_store_relaxed (adapt_count, + atomic_load_relaxed (adapt_count) - 1); } /* Use a normal lock here. */ diff --git a/sysdeps/unix/sysv/linux/x86/elision-trylock.c b/sysdeps/unix/sysv/linux/x86/elision-trylock.c index 65d9c18584..2d44f50426 100644 --- a/sysdeps/unix/sysv/linux/x86/elision-trylock.c +++ b/sysdeps/unix/sysv/linux/x86/elision-trylock.c @@ -36,8 +36,10 @@ __lll_trylock_elision (int *futex, short *adapt_count) return an error. */ _xabort (_ABORT_NESTED_TRYLOCK); - /* Only try a transaction if it's worth it. */ - if (*adapt_count <= 0) + /* Only try a transaction if it's worth it. See __lll_lock_elision for + why we need atomic accesses. Relaxed MO is sufficient because this is + just a hint. */ + if (atomic_load_relaxed (adapt_count) <= 0) { unsigned status; @@ -55,16 +57,18 @@ __lll_trylock_elision (int *futex, short *adapt_count) if (!(status & _XABORT_RETRY)) { /* Internal abort. No chance for retry. For future - locks don't try speculation for some time. */ - if (*adapt_count != aconf.skip_trylock_internal_abort) - *adapt_count = aconf.skip_trylock_internal_abort; + locks don't try speculation for some time. See above for MO. */ + if (atomic_load_relaxed (adapt_count) + != aconf.skip_lock_internal_abort) + atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort); } /* Could do some retries here. */ } else { - /* Lost updates are possible, but harmless. */ - (*adapt_count)--; + /* Lost updates are possible but harmless (see above). */ + atomic_store_relaxed (adapt_count, + atomic_load_relaxed (adapt_count) - 1); } return lll_trylock (*futex); diff --git a/sysdeps/x86/elide.h b/sysdeps/x86/elide.h index 8691e6673d..f7d5220c17 100644 --- a/sysdeps/x86/elide.h +++ b/sysdeps/x86/elide.h @@ -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; \ |