[MEH PATCH] fs/pipe: write to ->poll_usage only once

[Mateusz Guzik <mjguzik@gmail.com>]

Both GNU and BSD makes share a "token pipe" between their instances, as
a result a -j $BIGNUM invocation results in multicore perf problems in
the poll handler.

Avoiding the store will reduce it a little bit. However, the crux of the
problem is the locked queuing up in poll_wait().

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---

disclaimer: could not be bothered to bench this

 fs/pipe.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 9841648c9cf3..e37c79935ecb 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -664,7 +664,8 @@ pipe_poll(struct file *filp, poll_table *wait)
 	union pipe_index idx;
 
 	/* Epoll has some historical nasty semantics, this enables them */
-	WRITE_ONCE(pipe->poll_usage, true);
+	if (unlikely(!READ_ONCE(pipe->poll_usage)))
+		WRITE_ONCE(pipe->poll_usage, true);
 
 	/*
 	 * Reading pipe state only -- no need for acquiring the semaphore.
-- 
2.48.1