* Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > On 01/26, Andrew Morton wrote: > > > > On Mon, 26 Jan 2009 22:45:16 +0100 > > Ingo Molnar <mingo@xxxxxxx> wrote: > > > > > that would change the concept of execution but indeed it would be > > > interesting to try. It's outside the scope of late -rcs i guess, but > > > worthwile nevertheless. > > > > > > > Well it turns out that I was having a less-than-usually-senile moment: > > > > : commit b89deed32ccc96098bd6bc953c64bba6b847774f > > : Author: Oleg Nesterov <oleg@xxxxxxxxxx> > > : AuthorDate: Wed May 9 02:33:52 2007 -0700 > > : Commit: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxxxxxxxx> > > : CommitDate: Wed May 9 12:30:50 2007 -0700 > > : > > : implement flush_work() > > : > > : A basic problem with flush_scheduled_work() is that it blocks behind _all_ > > : presently-queued works, rather than just the work whcih the caller wants to > > : flush. If the caller holds some lock, and if one of the queued work happens > > : to want that lock as well then accidental deadlocks can occur. > > : > > : One example of this is the phy layer: it wants to flush work while holding > > : rtnl_lock(). But if a linkwatch event happens to be queued, the phy code will > > : deadlock because the linkwatch callback function takes rtnl_lock. > > : > > : So we implement a new function which will flush a *single* work - just the one > > : which the caller wants to free up. Thus we avoid the accidental deadlocks > > : which can arise from unrelated subsystems' callbacks taking shared locks. > > : > > : flush_work() non-blockingly dequeues the work_struct which we want to kill, > > : then it waits for its handler to complete on all CPUs. > > : > > : Add ->current_work to the "struct cpu_workqueue_struct", it points to > > : currently running "struct work_struct". When flush_work(work) detects > > : ->current_work == work, it inserts a barrier at the _head_ of ->worklist > > : (and thus right _after_ that work) and waits for completition. This means > > : that the next work fired on that CPU will be this barrier, or another > > : barrier queued by concurrent flush_work(), so the caller of flush_work() > > : will be woken before any "regular" work has a chance to run. > > : > > : When wait_on_work() unlocks workqueue_mutex (or whatever we choose to protect > > : against CPU hotplug), CPU may go away. But in that case take_over_work() will > > : move a barrier we queued to another CPU, it will be fired sometime, and > > : wait_on_work() will be woken. > > : > > : Actually, we are doing cleanup_workqueue_thread()->kthread_stop() before > > : take_over_work(), so cwq->thread should complete its ->worklist (and thus > > : the barrier), because currently we don't check kthread_should_stop() in > > : run_workqueue(). But even if we did, everything should be ok. > > > > > > Why isn't that working in this case?? > > Cough. Because that "flush_work()" was renamed to cancel_work_sync(). > Because it really cancells the work_struct if it can. > > Now we have flush_work() which does not cancel, but waits for completion > of the single work_struct. Of course, it can hang if the caller holds > the lock which can be taken by another work in that workqueue. > > Oleg. Andrew's suggestion does make sense though: for any not-in-progress worklet we can dequeue that worklet and execute it in the flushing context. [ And if that worklet cannot be dequeued because it's being processed then that's fine and we can wait on that single worklet, without waiting on any other 'unrelated' worklets. ] That does not help work_on_cpu() though: that facility really uses the fact that workqueues are implemented via per CPU threads - hence we cannot remove the worklet from the queue and execute it in the flushing context. Ingo -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
