diff options
Diffstat (limited to 'sysdeps/unix/sysv/linux/s390/elision-trylock.c')
| -rw-r--r-- | sysdeps/unix/sysv/linux/s390/elision-trylock.c | 79 |
1 files changed, 41 insertions, 38 deletions
diff --git a/sysdeps/unix/sysv/linux/s390/elision-trylock.c b/sysdeps/unix/sysv/linux/s390/elision-trylock.c index 3d5a994ad9..2eb9b36772 100644 --- a/sysdeps/unix/sysv/linux/s390/elision-trylock.c +++ b/sysdeps/unix/sysv/linux/s390/elision-trylock.c @@ -1,5 +1,5 @@ /* Elided pthread mutex trylock. - 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 @@ -19,7 +19,7 @@ #include <pthread.h> #include <pthreadP.h> #include <lowlevellock.h> -#include <htmintrin.h> +#include <htm.h> #include <elision-conf.h> #define aconf __elision_aconf @@ -30,15 +30,11 @@ int __lll_trylock_elision (int *futex, short *adapt_count) { - __asm__ __volatile__ (".machinemode \"zarch_nohighgprs\"\n\t" - ".machine \"all\"" - : : : "memory"); - /* Implement POSIX semantics by forbiding nesting elided trylocks. Sorry. After the abort the code is re-executed non transactional and if the lock was already locked return an error. */ - if (__builtin_tx_nesting_depth () > 0) + if (__libc_tx_nesting_depth () > 0) { /* Note that this abort may terminate an outermost transaction that was created outside glibc. @@ -46,49 +42,56 @@ __lll_trylock_elision (int *futex, short *adapt_count) them to use the default lock instead of retrying transactions until their try_tbegin is zero. */ - __builtin_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); + __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); + __builtin_unreachable (); } - /* Only try a transaction if it's worth it. */ - 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 && aconf.try_tbegin > 0) { - unsigned status; - - if (__builtin_expect - ((status = __builtin_tbegin ((void *)0)) == _HTM_TBEGIN_STARTED, 1)) + int status = __libc_tbegin ((void *) 0); + if (__glibc_likely (status == _HTM_TBEGIN_STARTED)) { - if (*futex == 0) + /* Check the futex to make sure nobody has touched it in the + mean time. This forces the futex into the cache and makes + sure the transaction aborts if another thread acquires the lock + concurrently. */ + if (__glibc_likely (atomic_load_relaxed (futex) == 0)) + /* Lock was free. Return to user code in a transaction. */ return 0; - /* Lock was busy. Fall back to normal locking. */ - /* Since we are in a non-nested transaction there is no need to abort, - which is expensive. */ - __builtin_tend (); + + /* Lock was busy. Fall back to normal locking. + This can be the case if e.g. adapt_count was decremented to zero + by a former release and another thread has been waken up and + acquired it. + Since we are in a non-nested transaction there is no need to abort, + which is expensive. Simply end the started transaction. */ + __libc_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. */ + different CPU, but that could happen anyway when the futex is + 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 + else if (status != _HTM_TBEGIN_TRANSIENT) { - if (status != _HTM_TBEGIN_TRANSIENT) - { - /* 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. */ - if (aconf.skip_trylock_internal_abort > 0) - *adapt_count = aconf.skip_trylock_internal_abort; - } + /* 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. */ + if (aconf.skip_trylock_internal_abort > 0) + *adapt_count = aconf.skip_trylock_internal_abort; } /* Could do some retries here. */ } - else - { - /* Lost updates are possible, but harmless. Due to races this might lead - to *adapt_count becoming less than zero. */ - (*adapt_count)--; - } + /* Use normal locking as fallback path if the transaction does not + succeed. */ return lll_trylock (*futex); } |