diff options
Diffstat (limited to 'sysdeps/unix/sysv/linux/x86/elision-lock.c')
| -rw-r--r-- | sysdeps/unix/sysv/linux/x86/elision-lock.c | 28 |
1 files changed, 20 insertions, 8 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. */ |