Re: [PATCH] md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

[Shaohua Li]

On Thu, Mar 10, 2016 at 06:19:42AM +1100, Neil Brown wrote:
> 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@xxxxxxxx
> >> Reported-by: Martin Svec <martin.svec@xxxxxxxx> (and others)
> >> Tested-by: Tom Weber <linux@xxxxxxxxxxxxxxxx>
> >> Fixes: 1b956f7a8f9a ("md/raid5: be more selective about distributing flags across batch.")
> >> Cc: stable@xxxxxxxxxxxxxxx (v4.1 and later)
> >> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> >
> > 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.

There is a handy WARN_ONCE(). It's like WARN_ON_ONCE() but allows printing exra info.

Thanks,
Shaohua