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

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Date:	Thu, 10 Mar 2016 06:19:42 +1100
From:	NeilBrown <neilb@...e.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	Shaohua Li <shli@...com>, linux-raid@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Martin Svec <martin.svec@...er.cz>
Subject: Re: [PATCH] md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

On Thu, Mar 10 2016, Shaohua Li wrote:

> On Wed, Mar 09, 2016 at 12:58:25PM +1100, Neil Brown wrote:
>> 
>> break_stripe_batch_list breaks up a batch and copies some flags from
>> the batch head to the members, preserving others.
>> 
>> It doesn't preserve or copy STRIPE_PREREAD_ACTIVE.  This is not
>> normally a problem as STRIPE_PREREAD_ACTIVE is cleared when a
>> stripe_head is added to a batch, and is not set on stripe_heads
>> already in a batch.
>> 
>> However there is no locking to ensure one thread doesn't set the flag
>> after it has just been cleared in another.  This does occasionally happen.
>> 
>> md/raid5 maintains a count of the number of stripe_heads with
>> STRIPE_PREREAD_ACTIVE set: conf->preread_active_stripes.  When
>> break_stripe_batch_list clears STRIPE_PREREAD_ACTIVE inadvertently
>> this could becomes incorrect and will never again return to zero.
>> 
>> md/raid5 delays the handling of some stripe_heads until
>> preread_active_stripes becomes zero.  So when the above mention race
>> happens, those stripe_heads become blocked and never progress,
>> resulting is write to the array handing.
>> 
>> So: change break_stripe_batch_list to preserve STRIPE_PREREAD_ACTIVE
>> in the members of a batch.
>> 
>> URL: https://bugzilla.kernel.org/show_bug.cgi?id=108741
>> URL: https://bugzilla.redhat.com/show_bug.cgi?id=1258153
>> URL: http://thread.gmane.org/5649C0E9.2030204@zoner.cz
>> Reported-by: Martin Svec <martin.svec@...er.cz> (and others)
>> Tested-by: Tom Weber <linux@...kyard.4t2.com>
>> Fixes: 1b956f7a8f9a ("md/raid5: be more selective about distributing flags across batch.")
>> Cc: stable@...r.kernel.org (v4.1 and later)
>> Signed-off-by: NeilBrown <neilb@...e.com>
>
> Applied, thanks Neil! I'll split the WARN_ON_ONCE and do it for each bit, so
> next time we can have clear clue.

I personally think that would look ugly and increase the in-line code
size for minimal gain.
If you want to make a change (which I'm in two minds about) I think it
would be much cleaner to do
  if (WARN_ON_ONCE(...)) printk(....);

Then at least the extra code will be out of line - not even loaded into
the instruction cache until needed.

Thanks,
NeilBrown

>
> Thanks,
> Shaohua
>
>> ---
>>  drivers/md/raid5.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index b4f02c9959f2..2e7d253be6ce 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -4236,7 +4236,6 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
>>  		WARN_ON_ONCE(sh->state & ((1 << STRIPE_ACTIVE) |
>>  					  (1 << STRIPE_SYNCING) |
>>  					  (1 << STRIPE_REPLACED) |
>> -					  (1 << STRIPE_PREREAD_ACTIVE) |
>>  					  (1 << STRIPE_DELAYED) |
>>  					  (1 << STRIPE_BIT_DELAY) |
>>  					  (1 << STRIPE_FULL_WRITE) |
>> @@ -4251,6 +4250,7 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
>>  					      (1 << STRIPE_REPLACED)));
>>  
>>  		set_mask_bits(&sh->state, ~(STRIPE_EXPAND_SYNC_FLAGS |
>> +					    (1 << STRIPE_PREREAD_ACTIVE) |
>>  					    (1 << STRIPE_DEGRADED)),
>>  			      head_sh->state & (1 << STRIPE_INSYNC));
>>  
>> -- 
>> 2.7.2
>> 

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