On Thu, Jan 03, 2013 at 08:27:20PM +0000, Adamson, Dros wrote: > > On Jan 3, 2013, at 3:11 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > On Thu, Jan 03, 2013 at 04:28:37PM +0000, Adamson, Dros wrote: > >> Hey, sorry for the late response, I've been on vacation. > >> > >> On Dec 21, 2012, at 6:45 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> > >> wrote: > >> > >>> On Fri, Dec 21, 2012 at 11:36:51PM +0000, Myklebust, Trond wrote: > >>>> Please reread what I said. There was no obvious circular > >>>> dependency, because nfsiod and rpciod are separate workqueues, both > >>>> created with WQ_MEM_RECLAIM. > >>> > >>> Oh, sorry, I read "rpciod" as "nfsiod"! > >>> > >>>> Dros' experience shows, however that a call to rpc_shutdown_client > >>>> in an nfsiod work item will deadlock with rpciod if the RPC task's > >>>> work item has been assigned to the same CPU as the one running the > >>>> rpc_shutdown_client work item. > >>>> > >>>> I can't tell right now if that is intentional (in which case the > >>>> WARN_ON in the rpc code is correct), or if it is a bug in the > >>>> workqueue code. For now, we're assuming the former. > >>> > >>> Well, Documentation/workqueue.txt says: > >>> > >>> "Each wq with WQ_MEM_RECLAIM set has an execution context > >>> reserved for it. If there is dependency among multiple work > >>> items used during memory reclaim, they should be queued to > >>> separate wq each with WQ_MEM_RECLAIM." > >> > >> The deadlock we were seeing was: > >> > >> - task A gets queued on rpciod workqueue and assigned kworker-0:0 - > >> task B gets queued on rpciod workqueue and assigned the same kworker > >> (kworker-0:0) - task A gets run, calls rpc_shutdown_client(), which > >> will loop forever waiting for task B to run rpc_async_release() - task > >> B will never run rpc_async_release() - it can't run until kworker-0:0 > >> is free, which won't happen until task A (rpc_shutdown_client) is done > >> > >> The same deadlock happened when we tried queuing the tasks on a > >> different workqueues -- queue_work() assigns the task to a kworker > >> thread and it's luck of the draw if it's the same kworker as task A. > >> We tried the different workqueue options, but nothing changed this > >> behavior. > >> > >> Once a work struct is queued, there is no way to back out of the > >> deadlock. From kernel/workqueue.c:insert_wq_barrier comment: > >> > >> * Currently, a queued barrier can't be canceled. This is because * > >> try_to_grab_pending() can't determine whether the work to be * > >> grabbed is at the head of the queue and thus can't clear LINKED * > >> flag of the previous work while there must be a valid next work * > >> after a work with LINKED flag set. > >> > >> So once a work struct is queued and there is an ordering dependency > >> (i.e. task A is before task B), there is no way to back task B out - > >> so we can't just call cancel_work() or something on task B in > >> rpc_shutdown_client. > >> > >> The root of our issue is that rpc_shutdown_client is never safe to > >> call from a workqueue context - it loops until there are no more > >> tasks, marking tasks as killed and waiting for them to be cleaned up > >> in each task's own workqueue context. Any tasks that have already been > >> assigned to the same kworker thread will never have a chance to run > >> this cleanup stage. > >> > >> When fixing this deadlock, Trond and I discussed changing how > >> rpc_shutdown_client works (making it workqueue safe), but Trond felt > >> that it'd be better to just not call it from a workqueue context and > >> print a warning if it is. > >> > >> IIRC we tried using different workqueues with WQ_MEM_RECLAIM (with no > >> success), but I'd argue that even if that did work it would still be > >> very easy to call rpc_shutdown_client from the wrong context and MUCH > >> harder to detect it. It's also unclear to me if setting rpciod > >> workqueue to WQ_MEM_RECLAIM would limit it to one kworker, etc... > > > > Both rpciod and nfsiod already set WQ_MEM_RECLAIM. > > Heh, oh yeah :) > > > > > But, right, looking at kernel/workqueue.c, it seems that the dedicated > > "rescuer" threads are invoked only in the case when work is stalled > > because a new worker thread isn't allocated quickly enough. > > > > So, what to do that's simplest enough that it would work for > > post-rc2/stable? I was happy having just a simple dedicated > > thread--these are only started when nfsd is, so there's no real thread > > proliferation problem. > > That should work fine. No, I meant I was happy having my workqueue handled by a single dedicated thread, which I don't see a trivial way to do any more. > The client went this way and spawns a new kthread before calling rpc_shutdown_client() from a workqueue context. This should happen very infrequently. I don't think I can do that, as I was really counting on being able to wait for the client to shut down. I'll think about it. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
