On 09/26/2013 06:23 PM, Anatol Pomozov wrote: > Hi > > On Thu, Sep 26, 2013 at 7:18 AM, Hannes Reinecke <hare@xxxxxxx> wrote: >> 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. > > Thanks for the info. > > We use modified version of "sbull" block device driver from GKH book. > We use it for testing block device startup/shutdown path + CFQ > manipulation. > > The sbull driver uses function > blk_init_queue(..., &dev->qlock); > > it passes lock as a second parameter and function makes the lock > swapping. According to your information passing lock to > blk_init_queue() considered as an "old non recommended way" so we > should modify our driver and avoid doing this. I'll take a look. > Yep. I would recommend to use the queue_lock directly whenever you need to pull requests off the request_queue, and use your own lock to protect any internal structures associated with handling the request internally. Yes, this _might_ involve some lock dancing between the queue_lock and the internal lock, but has the advantage that the internal lock typically it used far more often than the queue_lock itself. So you might even get some speed advantage here as you reduce contention on the queue_lock. 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 cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
