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    SubjectRe: wait_event and preemption in 2.6
    On Fri, Oct 08, 2004 at 01:45:10PM -0400, wrote:
    > I'm writing a device driver for PPC Linux and I'm using wait_event. It
    > seems to me that there is a potential race condition in wait_event when
    > preemption is turned on (2.6 kernel).
    > The scenario goes something like this: After the waiting process is
    > woken up and returns from schedule it goes to the top of the loop and
    > prepares to wait again (despite the condition being true). Then it will
    > check the condition and break out of the loop. But what if in-kernel
    > preemption occurs while it's doing that and another process is
    > immediately scheduled to run? Does the process sleep forever? Assume
    > that the event (say interrupt) that caused the original wakeup is a one
    > shot.
    > I'm probably missing something. I've googled for an answer and asked
    > some of my Linux friends but it's not clear. Thanks for any replies.
    > Please cc me.

    Hi Mike,
    Here is the answer Robert Love gave me to that very same question,
    over a year ago.......

    > On Mon, 2003-04-14 at 17:54, Joe Korty wrote:
    >> Is this analysis correct? If it is, perhaps there is an alternative
    >> to fixing these cases individually: make the TASK_INTERRUPTIBLE/
    >> TASK_UNINTERRUPTIBLE states block preemption. In which case the
    >> 'set_current_state(TASK_RUNNING)' macro would need to include the
    >> same preemption check as 'preemption_enable'.

    > Thankfully you are wrong or we would have some serious problems :)
    > See kernel/sched.c :: preempt_schedule() where we set p->preempt_count
    > Then see kernel/sched.c :: schedule() where we short-circuit the
    > remove-from-runqueue code if PREEMPT_ACTIVE is set.
    > Thus, it is safe to preempt regardless of the task's state. It will
    > eventually reschedule.
    > Robert Love
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