Hello! On Wed, Aug 17, 2016 at 05:29:32PM -0700, Piotr Sikora wrote:
> # HG changeset patch > # User Piotr Sikora <piotrsik...@google.com> > # Date 1471265532 25200 > # Mon Aug 15 05:52:12 2016 -0700 > # Node ID 40765d8ee4dd29089b0e60ed5b6099ac624e804e > # Parent 2f2ec92c3af93c11e195fb6d805df57518fede7c > Core: add ngx_atomic_store() and ngx_atomic_load(). > > Those functions must be used to prevent data races between > threads operating concurrently on the same variables. > > No performance loss measured in microbenchmarks on x86_64. > > No binary changes when compiled without __atomic intrinsics. > > Found with ThreadSanitizer. > > Signed-off-by: Piotr Sikora <piotrsik...@google.com> [...] > #define ngx_trylock(lock, value) > \ > - (*(lock) == 0 && ngx_atomic_cmp_set(lock, 0, value)) > + (ngx_atomic_load(lock) == 0 && ngx_atomic_cmp_set(lock, 0, value)) The "*(lock) == 0" check here is just an optimization, it only ensures that the lock is likely to succed. Atomicity is provided by the ngx_atomic_cmp_set() operation following the check. If the check returns a wrong result due to non-atomic load - this won't do any harm. The idea is that a quick-and-dirty non-atomic reading can be used to optimize things when the lock is already obtained by another process. This is especially important in spinlocks like in ngx_shmtx_lock(). The same is believed to apply to the other places changed as well. If you think there are places where atomic reading is critical - please highlight these particular places. -- Maxim Dounin http://nginx.org/ _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org http://mailman.nginx.org/mailman/listinfo/nginx-devel
