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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