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

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Message-ID: <20151030162443.GA31413@kernel.org>
Date:	Fri, 30 Oct 2015 09:25:32 -0700
From:	Shaohua Li <shli@...nel.org>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	Neil Brown <neilb@...e.de>,
	"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()

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.

Thanks,
Shaohua
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