diff options
Diffstat (limited to 'sysdeps/unix/sysv/linux/x86/force-elision.h')
| -rw-r--r-- | sysdeps/unix/sysv/linux/x86/force-elision.h | 44 |
1 files changed, 40 insertions, 4 deletions
diff --git a/sysdeps/unix/sysv/linux/x86/force-elision.h b/sysdeps/unix/sysv/linux/x86/force-elision.h index dd659c908f..61282d6678 100644 --- a/sysdeps/unix/sysv/linux/x86/force-elision.h +++ b/sysdeps/unix/sysv/linux/x86/force-elision.h @@ -18,9 +18,45 @@ /* Automatically enable elision for existing user lock kinds. */ #define FORCE_ELISION(m, s) \ - if (__pthread_force_elision \ - && (m->__data.__kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \ + if (__pthread_force_elision) \ { \ - mutex->__data.__kind |= PTHREAD_MUTEX_ELISION_NP; \ - s; \ + /* See concurrency notes regarding __kind in \ + struct __pthread_mutex_s in \ + sysdeps/nptl/bits/thread-shared-types.h. \ + \ + There are the following cases for the kind of a mutex \ + (The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \ + PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \ + only one of both flags can be set): \ + - both flags are not set: \ + This is the first lock operation for this mutex. Enable \ + elision as it is not enabled so far. \ + Note: It can happen that multiple threads are calling e.g. \ + pthread_mutex_lock at the same time as the first lock \ + operation for this mutex. Then elision is enabled for this \ + mutex by multiple threads. Storing with relaxed MO is enough \ + as all threads will store the same new value for the kind of \ + the mutex. But we have to ensure that we always use the \ + elision path regardless if this thread has enabled elision or \ + another one. \ + \ + - PTHREAD_MUTEX_ELISION_NP flag is set: \ + Elision was already enabled for this mutex by a previous lock \ + operation. See case above. Just use the elision path. \ + \ + - PTHREAD_MUTEX_NO_ELISION_NP flag is set: \ + Elision was explicitly disabled by pthread_mutexattr_settype. \ + Do not use the elision path. \ + Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \ + changed after mutex initialization. */ \ + int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \ + if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \ + { \ + mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \ + atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \ + } \ + if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \ + { \ + s; \ + } \ } |