[PATCH 2 of 2] Core: add ngx_atomic_store() and ngx_atomic_load()
mdounin at mdounin.ru
Mon Sep 12 15:50:03 UTC 2016
On Wed, Aug 17, 2016 at 05:29:32PM -0700, Piotr Sikora wrote:
> # HG changeset patch
> # User Piotr Sikora <piotrsikora at 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 <piotrsikora at 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.
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