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

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Message-ID: <877fm4vw6j.fsf@notabene.neil.brown.name>
Date:	Sat, 31 Oct 2015 09:16:52 +1100
From:	Neil Brown <neilb@...e.de>
To:	Shaohua Li <shli@...nel.org>, Roman Gushchin <klamm@...dex-team.ru>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid\@vger.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH] md/raid5: fix locking in handle_stripe_clean_event()

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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