On Mon, 26 Jan 2009 22:45:16 +0100 Ingo Molnar <mingo@xxxxxxx> wrote: > > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Mon, 26 Jan 2009 22:27:27 +0100 > > Ingo Molnar <mingo@xxxxxxx> wrote: > > > > > > > > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > So if it's generic it ought to be implemented in a generic way - not a > > > > > "dont use from any codepath that has a lock held that might > > > > > occasionally also be held in a keventd worklet". (which is a totally > > > > > unmaintainable proposition and which would just cause repeat bugs > > > > > again and again.) > > > > > > > > That's different. The core fault here lies in the keventd workqueue > > > > handling code. If we're flushing work A then we shouldn't go and > > > > block behind unrelated work B. > > > > > > the blocking is inherent in the concept of "a queue of worklets > > > handled by a single thread". > > > > > > If a worklet is blocked then all other work performed by that thread > > > is blocked as well. So by waiting on a piece of work in the queue, we > > > wait for all prior work queued up there as well. > > > > > > The only way to decouple that and to make them independent (and hence > > > independently flushable) is to create more parallel flows of > > > execution: i.e. by creating another thread (another workqueue). > > > > > > > Nope. As I said, the caller of flush_work() can detach the work item > > and run it directly. > > 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?? -- 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
