FAILED: patch "[PATCH] md/raid10: initialize r10_bio->read_slot before use." failed to apply to 5.4-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.4-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>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 93decc563637c4288380912eac0eb42fb246cc04 Mon Sep 17 00:00:00 2001
From: Kevin Vigor <kvigor@gmail.com>
Date: Fri, 6 Nov 2020 14:20:34 -0800
Subject: [PATCH] md/raid10: initialize r10_bio->read_slot before use.

In __make_request() a new r10bio is allocated and passed to
raid10_read_request(). The read_slot member of the bio is not
initialized, and the raid10_read_request() uses it to index an
array. This leads to occasional panics.

Fix by initializing the field to invalid value and checking for
valid value in raid10_read_request().

Cc: stable@vger.kernel.org
Signed-off-by: Kevin Vigor <kvigor@gmail.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index b7bca6703df8..3153183b7772 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1127,7 +1127,7 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
 	struct md_rdev *err_rdev = NULL;
 	gfp_t gfp = GFP_NOIO;
 
-	if (r10_bio->devs[slot].rdev) {
+	if (slot >= 0 && r10_bio->devs[slot].rdev) {
 		/*
 		 * This is an error retry, but we cannot
 		 * safely dereference the rdev in the r10_bio,
@@ -1508,6 +1508,7 @@ static void __make_request(struct mddev *mddev, struct bio *bio, int sectors)
 	r10_bio->mddev = mddev;
 	r10_bio->sector = bio->bi_iter.bi_sector;
 	r10_bio->state = 0;
+	r10_bio->read_slot = -1;
 	memset(r10_bio->devs, 0, sizeof(r10_bio->devs[0]) * conf->geo.raid_disks);
 
 	if (bio_data_dir(bio) == READ)