On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> > What is you do (under proper irqfd locking) something like:
> >
> > eventfd_ctx_read(ctx, 1, &cnt);
> > if (irqfd->cnt != cnt) {
> > irqfd->cnt = cnt;
> > schedule_work(&irqfd->inject);
> > }
> >
> >
> >
> >
> > > And deactivation deep down does this (from irqfd_cleanup_wq workqueue,
> > > so this is not under the spinlock):
> > >
> > > /*
> > > * Synchronize with the wait-queue and unhook ourselves to
> > > * prevent
> > > * further events.
> > > */
> > > B.
> > > remove_wait_queue(irqfd->wqh, &irqfd->wait);
> > >
> > > ....
> > >
> > > /*
> > > * It is now safe to release the object's resources
> > > */
> > > eventfd_ctx_put(irqfd->eventfd);
> > > kfree(irqfd);
> >
> > And:
> >
> > eventfd_ctx_read(ctx, 1, &irqfd->cnt);
>
>
> ->
>
> > remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >
> >
> >
> >
> > - Davide
>
> Yes, this is exactly what I wanted to do. So, here's the issue: if an
> event is signalled at point ->: after eventfd_ctx_read but before
> remove_wait_queue, then we inject interrupt but counter will be left
> non-zero and then when we unmask, we inject antoher, spurious interrupt.
>
> This is why I wanted to have eventfd_ctx_read not take wait queue head
> lock: then I could do:
>
> spin_lock_irqsave(&ctx->wqh.lock, flags);
> eventfd_ctx_read(ctx, 1, &irqfd->cnt);
> __remove_wait_queue(irqfd->wqh, &irqfd->wait);
> spin_lock_irqrestore(&ctx->wqh.lock, flags);
This is why I said "under proper irqfd locking".
- Davide
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