Re: FAILED: patch "[PATCH] io_uring/fdinfo: lock SQ thread while retrieving thread" failed to apply to 6.1-stable tree

[Jens Axboe <axboe@kernel.dk>]

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On 10/28/23 1:27 AM, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 6.1-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

This one applies to both 6.1 and 6.5-stable.

-- 
Jens Axboe


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From 740863413ea2d12ab31f1be3d23b01e246c2fa1c Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Sat, 28 Oct 2023 07:30:27 -0600
Subject: [PATCH] io_uring/fdinfo: lock SQ thread while retrieving thread
 cpu/pid

commit 7644b1a1c9a7ae8ab99175989bfc8676055edb46 upstream.

We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
dereference when the thread is cleared. Grab the SQPOLL data lock before
attempting to get the task cpu and pid for fdinfo, this ensures we have a
stable view of it.

Cc: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/fdinfo.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index 882bd56b01ed..ea2c2ded4e41 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -51,7 +51,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
 static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
 					  struct seq_file *m)
 {
-	struct io_sq_data *sq = NULL;
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
 	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
@@ -62,6 +61,7 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
 	unsigned int cq_shift = 0;
 	unsigned int sq_shift = 0;
 	unsigned int sq_entries, cq_entries;
+	int sq_pid = -1, sq_cpu = -1;
 	bool has_lock;
 	unsigned int i;
 
@@ -139,13 +139,19 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
 	has_lock = mutex_trylock(&ctx->uring_lock);
 
 	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
-		sq = ctx->sq_data;
-		if (!sq->thread)
-			sq = NULL;
+		struct io_sq_data *sq = ctx->sq_data;
+
+		if (mutex_trylock(&sq->lock)) {
+			if (sq->thread) {
+				sq_pid = task_pid_nr(sq->thread);
+				sq_cpu = task_cpu(sq->thread);
+			}
+			mutex_unlock(&sq->lock);
+		}
 	}
 
-	seq_printf(m, "SqThread:\t%d\n", sq ? task_pid_nr(sq->thread) : -1);
-	seq_printf(m, "SqThreadCpu:\t%d\n", sq ? task_cpu(sq->thread) : -1);
+	seq_printf(m, "SqThread:\t%d\n", sq_pid);
+	seq_printf(m, "SqThreadCpu:\t%d\n", sq_cpu);
 	seq_printf(m, "UserFiles:\t%u\n", ctx->nr_user_files);
 	for (i = 0; has_lock && i < ctx->nr_user_files; i++) {
 		struct file *f = io_file_from_index(&ctx->file_table, i);
-- 
2.42.0