On Thu, Jan 03, 2013 at 05:03:09PM -0500, Tejun Heo wrote: > Hello, guys. > > On Thu, Jan 03, 2013 at 04:28:37PM +0000, Adamson, Dros wrote: > > 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. > > Work items don't get assigned to workers on queueing. Idle workers > pick up work items. Oh, so that's why the case where we can't create a new worker is the only case we should need the rescuers for. Got it. I think. --b. > A work item is directly assigned to a specific > worker iff the worker is already executing that specific work item or > the new work item is "linked" to the one it's currently executing. > Currently, the only case where a linked work item is used is when > flushing which is guaranteed to not introduce dependency the other way > around. > > So, your diagnosis looks wrong to me. If such problem existed, we > would be seeing deadlocks all over the place. > > > Once a work struct is queued, there is no way to back out of the > > deadlock. From kernel/workqueue.c:insert_wq_barrier comment: > > Yes, there are. cancel_work[_sync]() do exactly that. > > > * 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. > > A *barrier* can't be canceled. A barrier is used only to flush work > items. The above comment means that we currently don't (or can't) > support canceling flush_work(). It has *nothing* to do with canceling > regular work items. You can cancel work items fine. > > > 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... > > It looks like you guys ended up in a weird place misled by wrong > analysis. Unless you require more than one concurrent execution on > the same workqueue, WQ_MEM_RECLAIM guarantees forward progress. It > won't deadlock because "a different work item is queued to the same > worker". The whole thing is designed *exactly* to avoid problems like > that. So, I'd strongly recommend looking again at why the deadlocks > are occurring. > > Thanks. > > -- > tejun > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- 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
