fs/pipe: remove unnecessary spinlock from pipe_write()

mainline inclusion
from mainline-v6.7-rc1
category: performance

This reverts commit 8df4412 ("pipe: Check for ring full inside of
the spinlock in pipe_write()") which was obsoleted by commit
c73be61 ("pipe: Add general notification queue support") because
now pipe_write() fails early with -EXDEV if there is a watch_queue.

Without a watch_queue, no notifications can be posted to the pipe and
mutex protection is enough, as can be seen in splice_pipe_to_pipe()
which does not use the spinlock either.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Message-Id: <20230921075755.1378787-3-max.kellermann@ionos.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
(cherry picked from commit dfaabf9)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Change-Id: I321d4c751c752c5e7e2c1a7b817b0e8cc14f1fe5

Author: MaxKellermann

fs/pipe.c

@@ -512,16 +512,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			 * it, either the reader will consume it or it'll still
 			 * be there for the next write.
 			 */
-			spin_lock_irq(&pipe->rd_wait.lock);
-
-			head = pipe->head;
-			if (pipe_full(head, pipe->tail, pipe->max_usage)) {
-				spin_unlock_irq(&pipe->rd_wait.lock);
-				continue;
-			}
-
 			pipe->head = head + 1;
-			spin_unlock_irq(&pipe->rd_wait.lock);
 
 			/* Insert it into the buffer array */
 			buf = &pipe->bufs[head & mask];