[PATCH -next] md: make rdev_addable usable for rcu mode

[PATCH -next] md: make rdev_addable usable for rcu mode

[Yang Erkun]

Our testcase trigger panic:

BUG: kernel NULL pointer dereference, address: 00000000000000e0
...
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 2 UID: 0 PID: 85 Comm: kworker/2:1 Not tainted 6.16.0+ #94
PREEMPT(none)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.1-2.fc37 04/01/2014
Workqueue: md_misc md_start_sync
RIP: 0010:rdev_addable+0x4d/0xf0
...
Call Trace:
 <TASK>
 md_start_sync+0x329/0x480
 process_one_work+0x226/0x6d0
 worker_thread+0x19e/0x340
 kthread+0x10f/0x250
 ret_from_fork+0x14d/0x180
 ret_from_fork_asm+0x1a/0x30
 </TASK>
Modules linked in: raid10
CR2: 00000000000000e0
---[ end trace 0000000000000000 ]---
RIP: 0010:rdev_addable+0x4d/0xf0

md_spares_need_change in md_start_sync will call rdev_addable which
protected by rcu_read_lock/rcu_read_unlock. This rcu context will help
protect rdev won't be released, but rdev->mddev will be set to NULL
before we call synchronize_rcu in md_kick_rdev_from_array. Fix this by
using READ_ONCE and check does rdev->mddev still alive.

Fixes: bc08041b32ab ("md: suspend array in md_start_sync() if array need reconfiguration")
Fixes: 570b9147deb6 ("md: use RCU lock to protect traversal in md_spares_need_change()")
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
---
 drivers/md/md.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 046fe85c76fe..adc5f0b48a00 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9421,6 +9421,12 @@ static bool rdev_is_spare(struct md_rdev *rdev)
 
 static bool rdev_addable(struct md_rdev *rdev)
 {
+	struct mddev *mddev;
+
+	mddev = READ_ONCE(rdev->mddev);
+	if (!mddev)
+		return false;
+
 	/* rdev is already used, don't add it again. */
 	if (test_bit(Candidate, &rdev->flags) || rdev->raid_disk >= 0 ||
 	    test_bit(Faulty, &rdev->flags))
@@ -9431,7 +9437,7 @@ static bool rdev_addable(struct md_rdev *rdev)
 		return true;
 
 	/* Allow to add if array is read-write. */
-	if (md_is_rdwr(rdev->mddev))
+	if (md_is_rdwr(mddev))
 		return true;
 
 	/*
-- 
2.39.2

Re: [PATCH -next] md: make rdev_addable usable for rcu mode

[yukuai]

在 2025/7/31 19:45, Yang Erkun 写道:
> Our testcase trigger panic:
>
> BUG: kernel NULL pointer dereference, address: 00000000000000e0
> ...
> Oops: Oops: 0000 [#1] SMP NOPTI
> CPU: 2 UID: 0 PID: 85 Comm: kworker/2:1 Not tainted 6.16.0+ #94
> PREEMPT(none)
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.16.1-2.fc37 04/01/2014
> Workqueue: md_misc md_start_sync
> RIP: 0010:rdev_addable+0x4d/0xf0
> ...
> Call Trace:
>   <TASK>
>   md_start_sync+0x329/0x480
>   process_one_work+0x226/0x6d0
>   worker_thread+0x19e/0x340
>   kthread+0x10f/0x250
>   ret_from_fork+0x14d/0x180
>   ret_from_fork_asm+0x1a/0x30
>   </TASK>
> Modules linked in: raid10
> CR2: 00000000000000e0
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:rdev_addable+0x4d/0xf0
>
> md_spares_need_change in md_start_sync will call rdev_addable which
> protected by rcu_read_lock/rcu_read_unlock. This rcu context will help
> protect rdev won't be released, but rdev->mddev will be set to NULL
> before we call synchronize_rcu in md_kick_rdev_from_array. Fix this by
> using READ_ONCE and check does rdev->mddev still alive.
>
> Fixes: bc08041b32ab ("md: suspend array in md_start_sync() if array need reconfiguration")
> Fixes: 570b9147deb6 ("md: use RCU lock to protect traversal in md_spares_need_change()")
> Signed-off-by: Yang Erkun<yangerkun@huawei.com>
> ---
>   drivers/md/md.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
Applied to md-6.17

Thanks
Kuai