linux-kernel - [RFC PATCH 2/3] fs/splice: Atomically read pipe->{head,tail} in opipe

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Message-ID: <20250306113924.20004-3-kprateek.nayak@amd.com>
Date: Thu, 6 Mar 2025 11:39:23 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>, Oleg Nesterov
	<oleg@...hat.com>, Miklos Szeredi <miklos@...redi.hu>, Alexander Viro
	<viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, "Andrew
 Morton" <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
CC: Jan Kara <jack@...e.cz>, "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mateusz Guzik <mjguzik@...il.com>, "Gautham R. Shenoy"
	<gautham.shenoy@....com>, Rasmus Villemoes <ravi@...vas.dk>,
	<Neeraj.Upadhyay@....com>, <Ananth.narayan@....com>, Swapnil Sapkal
	<swapnil.sapkal@....com>, K Prateek Nayak <kprateek.nayak@....com>
Subject: [RFC PATCH 2/3] fs/splice: Atomically read pipe->{head,tail} in opipe_prep()

opipe_prep() checks pipe_full() before taking the "pipe->mutex". Use the
newly introduced "pipe->head_tail" member to read the head and the tail
atomically and not miss any updates between the reads.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 fs/splice.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/splice.c b/fs/splice.c
index 28cfa63aa236..e51f33aca032 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1682,13 +1682,14 @@ static int ipipe_prep(struct pipe_inode_info *pipe, unsigned int flags)
  */
 static int opipe_prep(struct pipe_inode_info *pipe, unsigned int flags)
 {
+	union pipe_index idx = { .head_tail = READ_ONCE(pipe->head_tail) };
 	int ret;
 
 	/*
 	 * Check pipe occupancy without the inode lock first. This function
 	 * is speculative anyways, so missing one is ok.
 	 */
-	if (!pipe_full(pipe->head, pipe->tail, pipe->max_usage))
+	if (!pipe_full(idx.head, idx.tail, READ_ONCE(pipe->max_usage)))
 		return 0;
 
 	ret = 0;
-- 
2.43.0