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. The client went this way and spawns a new kthread before calling rpc_shutdown_client() from a workqueue context. This should happen very infrequently. -dros > > I'll go quietly weep for a little while and then think about it some > more.... -- 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
