On 09/26/2013 03:54 PM, Tejun Heo wrote: > Hello, (cc'ing linux-scsi) > > On Wed, Sep 25, 2013 at 01:37:51PM -0700, Anatol Pomozov wrote: >> Hi >> >> On Wed, Sep 4, 2013 at 9:07 AM, Tejun Heo <tj@xxxxxxxxxx> wrote: >>> Hello, >>> >>> On Wed, Sep 04, 2013 at 08:45:33AM -0700, Anatol Pomozov wrote: >>>> I am not an expect in block code, so I have a few questions here: >>>> >>>> - are we sure that this operation is atomic? What if blkg->q becomes >>>> dead right after we checked it, and blkg->q->queue_lock got invalid so >>>> we have the same crash as before? >>> >>> request_queue lock switching is something inherently broken in block >>> layer. It's unsalvageable. >> >> Fully agree. The problem that request_queue->queue_lock is a shared >> resource that concurrently modified/accessed. In this case (when one >> thread changes, another thread access it) we need synchronization to >> prevent race conditions. So we need a spin_lock to access queue_lock >> spin_lock, otherwise we have a crash like one above... >> >>> Maybe we can drop lock switching once blk-mq is fully merged. >> >> Could you please provide more information about it? What is the timeline? > > I have no idea. Hopefully, not too far out. Jens would have better > idea. > >> If there is an easy way to fix the race condition I would like to >> help. Please give me some pointer what direction I should move. > > The first step would be identifying who are actually making use of > lock switching, why and how much difference it would make for them to > not do that. > Typically, the lock is being used by the block drivers to synchronize access between some internal data structures and the request queue itself. You don't actually _need_ to do it that way, but removing the lock switching would involve quite some redesign of these drivers. Give that most of the are rather oldish I really wouldn't want to touch them. However, none of the modern devices should be using this lock switching, so I would just ignore it. EG SCSI most definitely doesn't use it. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
