Re: [PATCH] md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list
Date: Wed Mar 09 2016 - 14:20:12 EST
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.
>> 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));
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