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To:     Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Subject: [PATCH] pipe: honor IOCB_NOWAIT
Message-ID: <cedfa436-47a3-7cbc-1948-75d0e28cfdc5@kernel.dk>
Date:   Mon, 7 Sep 2020 09:21:02 -0600
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Pipe only looks at O_NONBLOCK for non-blocking operation, which means that
io_uring can't easily poll for it or attempt non-blocking issues. Check for
IOCB_NOWAIT in locking the pipe for reads and writes, and ditto when we
decide on whether or not to block or return -EAGAIN.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

If this is acceptable, then I can add S_ISFIFO to the whitelist on file
descriptors we can IOCB_NOWAIT try for, then poll if we get -EAGAIN
instead of using thread offload.

diff --git a/fs/pipe.c b/fs/pipe.c
index 60dbee457143..3cee28e35985 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -82,6 +82,13 @@ void pipe_unlock(struct pipe_inode_info *pipe)
 }
 EXPORT_SYMBOL(pipe_unlock);
 
+static inline bool __pipe_lock_nonblock(struct pipe_inode_info *pipe)
+{
+	if (mutex_trylock(&pipe->mutex))
+		return true;
+	return false;
+}
+
 static inline void __pipe_lock(struct pipe_inode_info *pipe)
 {
 	mutex_lock_nested(&pipe->mutex, I_MUTEX_PARENT);
@@ -244,7 +251,12 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		return 0;
 
 	ret = 0;
-	__pipe_lock(pipe);
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!__pipe_lock_nonblock(pipe))
+			return -EAGAIN;
+	} else {
+		__pipe_lock(pipe);
+	}
 
 	/*
 	 * We only wake up writers if the pipe was full when we started
@@ -344,7 +356,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			break;
 		if (ret)
 			break;
-		if (filp->f_flags & O_NONBLOCK) {
+		if ((filp->f_flags & O_NONBLOCK) ||
+		    (iocb->ki_flags & IOCB_NOWAIT)) {
 			ret = -EAGAIN;
 			break;
 		}
@@ -432,7 +445,12 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	if (unlikely(total_len == 0))
 		return 0;
 
-	__pipe_lock(pipe);
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!__pipe_lock_nonblock(pipe))
+			return -EAGAIN;
+	} else {
+		__pipe_lock(pipe);
+	}
 
 	if (!pipe->readers) {
 		send_sig(SIGPIPE, current, 0);
@@ -554,7 +572,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			continue;
 
 		/* Wait for buffer space to become available. */
-		if (filp->f_flags & O_NONBLOCK) {
+		if ((filp->f_flags & O_NONBLOCK) ||
+		    (iocb->ki_flags & IOCB_NOWAIT)) {
 			if (!ret)
 				ret = -EAGAIN;
 			break;

-- 
Jens Axboe