linux-kernel - Re: [PATCH] md/raid5: fix locking in handle_stripe_clean

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Date:	Sat, 31 Oct 2015 15:25:42 +0300
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Neil Brown <neilb@...e.de>, Shaohua Li <shli@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH] md/raid5: fix locking in handle_stripe_clean_event()

Ok, thank you for clarifications!

--
Roman


31.10.2015, 01:17, "Neil Brown" <neilb@...e.de>:
> On Sat, Oct 31 2015, Shaohua Li wrote:
>
>>  On Fri, Oct 30, 2015 at 05:02:47PM +0300, Roman Gushchin wrote:
>>>  > Isn't the 4.1 fix just:
>>>  >
>>>  > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>>>  > index e5befa356dbe..6e4350a78257 100644
>>>  > --- a/drivers/md/raid5.c
>>>  > +++ b/drivers/md/raid5.c
>>>  > @@ -3522,16 +3522,16 @@ returnbi:
>>>  >                   * no updated data, so remove it from hash list and the stripe
>>>  >                   * will be reinitialized
>>>  >                   */
>>>  > - spin_lock_irq(&conf->device_lock);
>>>  >  unhash:
>>>  > + spin_lock_irq(conf->hash_locks + sh->hash_lock_index);
>>>  >                  remove_hash(sh);
>>>  > + spin_unlock_irq(conf->hash_locks + sh->hash_lock_index);
>>>  >                  if (head_sh->batch_head) {
>>>  >                          sh = list_first_entry(&sh->batch_list,
>>>  >                                                struct stripe_head, batch_list);
>>>  >                          if (sh != head_sh)
>>>  >                                          goto unhash;
>>>  >                  }
>>>  > - spin_unlock_irq(&conf->device_lock);
>>>  >                  sh = head_sh;
>>>  >
>>>  >                  if (test_bit(STRIPE_SYNC_REQUESTED, &sh->state))
>>>  >
>>>  > ??
>>>
>>>  In my opion, this patch looks correct, although it seems to me, that there is an another issue here.
>>>
>>>  >                  if (head_sh->batch_head) {
>>>  >                          sh = list_first_entry(&sh->batch_list,
>>>  >                                                struct stripe_head, batch_list);
>>>  >                          if (sh != head_sh)
>>>  >                                          goto unhash;
>>>  >                  }
>>>
>>>  With a patch above this code will be executed without taking any locks. It it correct?
>>>  In my opinion, we need to take at least sh->stripe_lock, which protects sh->batch_head.
>>>  Or do I miss something?
>>>
>>>  If you want, we can handle this issue separately.
>>
>>  The batch_list list doesn't need the protection. Only the remove_hash() need it.
>
> Yes, that's my understanding too. The key to understanding is that
> comment you (helpfully!) put in clear_batch_ready():
>
>         /*
>          * BATCH_READY is cleared, no new stripes can be added.
>          * batch_list can be accessed without lock
>          */
>
> I'll wrangle some patches...
>
> Thanks,
> NeilBrown
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