linux-kernel - [RFC PATCH 05/21] pipe: Conditionalise wakeup in pipe

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Date:   Tue, 15 Oct 2019 22:48:34 +0100
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     dhowells@...hat.com, Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>, dhowells@...hat.com,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 05/21] pipe: Conditionalise wakeup in pipe_read()

Only do a wakeup in pipe_read() if we made space in a completely full
buffer.  The producer shouldn't be waiting on pipe->wait otherwise.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/pipe.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 08af7e7bbea2..0d25cb090a03 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -327,11 +327,13 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				spin_lock_irq(&pipe->wait.lock);
 				tail++;
 				pipe_commit_read(pipe, tail);
-				do_wakeup = 0;
-				prelocked_wake_up_interruptible_sync_poll(
-					&pipe->wait, EPOLLOUT | EPOLLWRNORM);
+				do_wakeup = 1;
+				if (head - (tail - 1) == pipe->max_usage)
+					prelocked_wake_up_interruptible_sync_poll(
+						&pipe->wait, EPOLLOUT | EPOLLWRNORM);
 				spin_unlock_irq(&pipe->wait.lock);
-				kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
+				if (head - (tail - 1) == pipe->max_usage)
+					kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 			}
 			total_len -= chars;
 			if (!total_len)
@@ -360,11 +362,6 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				ret = -ERESTARTSYS;
 			break;
 		}
-		if (do_wakeup) {
-			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
- 			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-			do_wakeup = 0;
-		}
 		pipe_wait(pipe);
 	}
 	__pipe_unlock(pipe);