Messages in this thread | | | Date | Tue, 9 Jan 2007 18:55:04 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] reimplement flush_workqueue() |
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On 01/09, Srivatsa Vaddagiri wrote: > > On Fri, Dec 29, 2006 at 08:18:27PM +0300, Oleg Nesterov wrote: > > Remove ->remove_sequence, ->insert_sequence, and ->work_done from struct > > cpu_workqueue_struct. To implement flush_workqueue() we can queue a barrier > > work on each CPU and wait for its completition. > > Oleg, > Because of this change, was curious to know if this is possible: > > > CPU0 CPU1 > (Thread0) > > flush_workqueue() > queue_work(W1) > flush_cpu_workqueue(cpu1) > insert_barrier(B1) > wait_on_completion(); > > run_workqueue() > W1.func(); > flush_workqueue(); > B1.func(); <- wakes Thread0 > > The intention of barrier B1 was to wait untill W1 was -complete-. If > W1.func()->....->something() were to call flush_workqueue on the same > workqueue, then we would be returning from the barrier prematurely.
But there is nothing new?
insert_sequence = remove_sequence = 0.
queue_work(W1) sets insert_sequence = 1.
flush_cpu_workqueue(cpu1): wait until remove_sequence >= 1
Now suppose antother thread adds a work to cpu1 before W1.func() calls flush_cpu_workqueue(cpu1). insert_sequence == 2.
When W1.func() does flush_workqueue(), run_workqueue() fires that work, increments remove_sequence to 1 and wakes up Thread0.
In other words: currently flush_cpu_workqueue() waits until N works form the queue will be flushed. If some work also does flush_workqueue()->run_workqueue(), it just needs to execute one "extra" work to confuse the first flush_cpu_workqueue().
Oleg.
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