diff options
| -rw-r--r-- | ChangeLog | 7 | ||||
| -rw-r--r-- | sysdeps/unix/sysv/linux/s390/elision-lock.c | 25 | ||||
| -rw-r--r-- | sysdeps/unix/sysv/linux/s390/elision-trylock.c | 14 |
3 files changed, 34 insertions, 12 deletions
@@ -1,3 +1,10 @@ +2016-12-20 Stefan Liebler <stli@linux.vnet.ibm.com> + + * sysdeps/unix/sysv/linux/s390/elision-lock.c + (__lll_lock_elision): Use atomics to load / store adapt_count. + * sysdeps/unix/sysv/linux/s390/elision-trylock.c + (__lll_trylock_elision): Likewise. + 2016-12-20 Florian Weimer <fweimer@redhat.com> Do not require memset elimination in explicit_bzero test. diff --git a/sysdeps/unix/sysv/linux/s390/elision-lock.c b/sysdeps/unix/sysv/linux/s390/elision-lock.c index ecb507e989..1876d2128d 100644 --- a/sysdeps/unix/sysv/linux/s390/elision-lock.c +++ b/sysdeps/unix/sysv/linux/s390/elision-lock.c @@ -45,11 +45,18 @@ 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) { /* Lost updates are possible, but harmless. Due to races this might lead to *adapt_count becoming less than zero. */ - (*adapt_count)--; + atomic_store_relaxed (adapt_count, + atomic_load_relaxed (adapt_count) - 1); goto use_lock; } @@ -74,8 +81,10 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) /* In a non-nested transaction there is no need to abort, which is expensive. */ __builtin_tend (); + /* Don't try to use transactions for the next couple of times. + See above for why relaxed MO is sufficient. */ if (aconf.skip_lock_busy > 0) - *adapt_count = aconf.skip_lock_busy; + atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); goto use_lock; } else /* nesting depth is > 1 */ @@ -101,18 +110,20 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) /* A persistent abort (cc 1 or 3) indicates that a retry is probably futile. Use the normal locking now and for the next couple of calls. - Be careful to avoid writing to the lock. */ + Be careful to avoid writing to the lock. See above for why + relaxed MO is sufficient. */ if (aconf.skip_lock_internal_abort > 0) - *adapt_count = aconf.skip_lock_internal_abort; + atomic_store_relaxed (adapt_count, + aconf.skip_lock_internal_abort); goto use_lock; } } } /* Same logic as above, but for for a number of temporary failures in a - row. */ + row. See above for why relaxed MO is sufficient. */ if (aconf.skip_lock_out_of_tbegin_retries > 0 && aconf.try_tbegin > 0) - *adapt_count = aconf.skip_lock_out_of_tbegin_retries; + atomic_store_relaxed (adapt_count, aconf.skip_lock_out_of_tbegin_retries); use_lock: return LLL_LOCK ((*futex), private); diff --git a/sysdeps/unix/sysv/linux/s390/elision-trylock.c b/sysdeps/unix/sysv/linux/s390/elision-trylock.c index 3d5a994ad9..a3252b83ce 100644 --- a/sysdeps/unix/sysv/linux/s390/elision-trylock.c +++ b/sysdeps/unix/sysv/linux/s390/elision-trylock.c @@ -49,8 +49,10 @@ __lll_trylock_elision (int *futex, short *adapt_count) __builtin_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); } - /* 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; @@ -65,9 +67,10 @@ __lll_trylock_elision (int *futex, short *adapt_count) __builtin_tend (); /* Note: Changing the adapt_count here might abort a transaction on a different cpu, but that could happen anyway when the futex is - acquired, so there's no need to check the nesting depth here. */ + acquired, so there's no need to check the nesting depth here. + See above for why relaxed MO is sufficient. */ if (aconf.skip_lock_busy > 0) - *adapt_count = aconf.skip_lock_busy; + atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); } else { @@ -87,7 +90,8 @@ __lll_trylock_elision (int *futex, short *adapt_count) { /* Lost updates are possible, but harmless. Due to races this might lead to *adapt_count becoming less than zero. */ - (*adapt_count)--; + atomic_store_relaxed (adapt_count, + atomic_load_relaxed (adapt_count) - 1); } return lll_trylock (*futex); |