linux-kernel - Re: [PATCH] md/raid5: preserve STRIPE_ON_UNPLUG_LIST in break_stripe_batch

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Date:   Thu, 31 Aug 2017 22:38:57 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Dennis Yang <dennisyang@...p.com>
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md/raid5: preserve STRIPE_ON_UNPLUG_LIST in
 break_stripe_batch_list

On Mon, Aug 28, 2017 at 08:01:59PM +0800, Dennis Yang wrote:
> break_stripe_batch_list() did not preserve STRIPE_ON_UNPLUG_LIST which is
> set when a stripe_head gets queued to the stripe_head list maintained by
> raid5_plug_cb and waiting for releasing after blk_unplug().
> 
> In release_stripe_plug(), if a stripe_head has its STRIPE_ON_UNPLUG_LIST
> set, it indicates that this stripe_head is already in the raid5_plug_cb
> list and release_stripe() would be called instead to drop a reference
> count. Otherwise, the STRIPE_ON_UNPLUG_LIST bit would be set for this
> stripe_head and it will get queued into the raid5_plug_cb list.
> 
> Without preserving STRIPE_ON_UNPLUG_LIST in break_stripe_batch_list(),
> a stripe_head could be re-added to the raid5_plug_cb list while it is
> currently on that list and waiting to be released. This would mess up
> the raid5_plug_cb and leads to soft/hard lockup in raid5_unplug() or
> kernel crash.

Did you observe this? From my understanding, this is impossible. If a stripe is
in batch list (that's why we have break_stripe_batch_list), add_stripe_bio will
bail out (because sh->batch_head != NULL), so the stripe is never added into
plug list, hence has the STRIPE_ON_UNPLUG_LIST bit set. Am I missing anything?

Thanks,
Shaohua
 
> Signed-off-by: Dennis Yang <dennisyang@...p.com>
> ---
>  drivers/md/raid5.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index e92dd2d..faf3cfd 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -4611,7 +4611,8 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
>  
>  		set_mask_bits(&sh->state, ~(STRIPE_EXPAND_SYNC_FLAGS |
>  					    (1 << STRIPE_PREREAD_ACTIVE) |
> -					    (1 << STRIPE_DEGRADED)),
> +					    (1 << STRIPE_DEGRADED) |
> +					    (1 << STRIPE_ON_UNPLUG_LIST)),
>  			      head_sh->state & (1 << STRIPE_INSYNC));
>  
>  		sh->check_state = head_sh->check_state;
> -- 
> 1.9.1
>