On 11/20/2015 06:13 AM, Chris Mason wrote:
> On Thu, Nov 19, 2015 at 05:49:37PM -0800, Liu Bo wrote:
>> while xfstesting, this bug[1] is spotted by both btrfs/061 and btrfs/063,
>> so those sub-stripe writes are gatherred into plug callback list and
>> hopefully we can have a full stripe writes.
>>
>> However, while processing these plugged callbacks, it's within an atomic
>> context which is provided by blk_sq_make_request() because of a get_cpu()
>> in blk_mq_get_ctx().
>>
>> This changes to always use btrfs_rmw_helper to complete the pending writes.
>>
>
> Thanks Liu, but MD raid has the same troubles, we're not atomic in our unplugs.
>
> Jens?

Yeah, blk-mq does have preemption disabled when it flushes, for the 
single queue setup. That's a bug. Attached is an untested patch that 
should fix it, can you try it?

I'll rework this to be a proper patch, not convinced we want to add the 
new request before flush, that might destroy merging opportunities. I'll 
unify the mq/sq parts.

-- 
Jens Axboe