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unix: yield cpu when spinlocking on async handle

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Fixes: https://github.com/libuv/libuv/issues/2769
PR-URL: https://github.com/libuv/libuv/pull/2772
Reviewed-By: Anna Henningsen <anna@addaleax.net>
This commit is contained in:
Ben Noordhuis 2020-04-22 12:28:11 +02:00
parent 72fe3543fe
commit a9d9d4ea1b
1 changed files with 21 additions and 8 deletions
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29
src/unix/async.c
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@ -32,6 +32,7 @@
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sched.h> /* sched_yield() */
#ifdef __linux__
#include <sys/eventfd.h>
@ -81,20 +82,32 @@ int uv_async_send(uv_async_t* handle) {
/* Only call this from the event loop thread. */
static int uv__async_spin(uv_async_t* handle) {
int i;
int rc;
for (;;) {
/* rc=0 -- handle is not pending.
* rc=1 -- handle is pending, other thread is still working with it.
* rc=2 -- handle is pending, other thread is done.
/* 997 is not completely chosen at random. It's a prime number, acyclical
* by nature, and should therefore hopefully dampen sympathetic resonance.
*/
rc = cmpxchgi(&handle->pending, 2, 0);
for (i = 0; i < 997; i++) {
/* rc=0 -- handle is not pending.
* rc=1 -- handle is pending, other thread is still working with it.
* rc=2 -- handle is pending, other thread is done.
*/
rc = cmpxchgi(&handle->pending, 2, 0);
if (rc != 1)
return rc;
if (rc != 1)
return rc;
/* Other thread is busy with this handle, spin until it's done. */
cpu_relax();
/* Other thread is busy with this handle, spin until it's done. */
cpu_relax();
}
/* Yield the CPU. We may have preempted the other thread while it's
* inside the critical section and if it's running on the same CPU
* as us, we'll just burn CPU cycles until the end of our time slice.
*/
sched_yield();
}
}