Re: wait_event and preemption in 2.6

[Joe Korty <joe.korty@ccur.com>]

On Fri, Oct 08, 2004 at 01:45:10PM -0400, michael_soulier@mitel.com 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
> to PREEMPT_ACTIVE.
>
> 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