Messages in this thread |  | | | Date | Fri, 26 Jun 2009 10:32:17 -0700 (PDT) | | From | Davide Libenzi <> | | Subject | Re: [PATCH] net: fix race in the receive/select |
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On Fri, 26 Jun 2009, Oleg Nesterov wrote:
> And if we remove waitqueue_active() in xxx_update(), then lock/unlock is > not needed too. > > If xxx_poll() takes q->lock first, it can safely miss the changes in ->status > and schedule(): xxx_update() will take q->lock, notice the sleeper and wake > it up (ok, it will set ->triggered but this doesn't matter). > > If xxx_update() takes q->lock first, xxx_poll() must see the changes in > status after poll_wait()->unlock(&q->lock) (in fact, after lock, not unlock).
Sure. The snippet above was just to show what typically the code does, not a suggestion on how to solve the socket case. But yeah, the problem in this case is the waitqueue_active() call. Without that, the wait queue lock/unlock in poll_wait() and the one in wake_up() guarantees the necessary barriers. Some might argue the costs of the lock/unlock of q->lock, and wonder if MBs are a more efficient solution. This is something I'm not going into. To me, it just looked not right having cross-matching MB in different subsystems.
- Davide
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