Re: [PATCH] md/raid5: fix locking in handle_stripe_clean_event()

[Neil Brown]

On Wed, Oct 28 2015, Roman Gushchin wrote:

> After commit 566c09c53455 ("raid5: relieve lock contention in get_active_stripe()")
> __find_stripe() is called under conf->hash_locks + hash.
> But handle_stripe_clean_event() calls remove_hash() under
> conf->device_lock.
>
> Under some cirscumstances the hash chain can be circuited,
> and we get an infinite loop with disabled interrupts and locked hash
> lock in __find_stripe(). This leads to hard lockup on multiple CPUs
> and following system crash.
>
> I was able to reproduce this behavior on raid6 over 6 ssd disks.
> The devices_handle_discard_safely option should be set to enable trim
> support. The following script was used:
>
> for i in `seq 1 32`; do
>     dd if=/dev/zero of=large$i bs=10M count=100 &
> done
>
> Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
> Cc: Neil Brown <neilb@xxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: linux-raid@xxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.10 - 3.19

Hi Roman,
 thanks for reporting this and providing a fix.

I'm a bit confused by that stable range: 3.10 - 3.19

The commit you identify as introducing the bug was added in 3.13, so
presumably 3.10, 3.11, 3.12 are not affected.
Also the bug is still present in mainline, so 4.0, 4.1, 4.2 are also
affected, though the patch needs to be revised a bit for 4.1 and later.

Does that match your understanding?  Or is there something that I am
missing?

Thanks,
NeilBrown

> ---
>  drivers/md/raid5.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index e421016..5fa7549 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -3060,6 +3060,8 @@ static void handle_stripe_clean_event(struct r5conf *conf,
>  		}
>  	if (!discard_pending &&
>  	    test_bit(R5_Discard, &sh->dev[sh->pd_idx].flags)) {
> +		int hash = sh->hash_lock_index;
> +
>  		clear_bit(R5_Discard, &sh->dev[sh->pd_idx].flags);
>  		clear_bit(R5_UPTODATE, &sh->dev[sh->pd_idx].flags);
>  		if (sh->qd_idx >= 0) {
> @@ -3073,9 +3075,9 @@ static void handle_stripe_clean_event(struct r5conf *conf,
>  		 * no updated data, so remove it from hash list and the stripe
>  		 * will be reinitialized
>  		 */
> -		spin_lock_irq(&conf->device_lock);
> +		spin_lock_irq(conf->hash_locks + hash);
>  		remove_hash(sh);
> -		spin_unlock_irq(&conf->device_lock);
> +		spin_unlock_irq(conf->hash_locks + hash);
>  		if (test_bit(STRIPE_SYNC_REQUESTED, &sh->state))
>  			set_bit(STRIPE_HANDLE, &sh->state);
>  
> -- 
> 2.4.3
>
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