Re: PROBLEM: Bug in __pollwait() can cause select() and poll() tohang in

[Ray Bryant <raybry@sgi.com>]

Manfred Spraul wrote:
> 
> Hi Ray,
> 
> your bug description seems to be correct, but the fix is wrong:
> If the allocation is for the 2nd page of wait queue heads, then
> "current->state = TASK_INTERRUPTIBLE" can lead to lost wakeups, if an fd
> that is stored in the first page gets ready during the allocation.

Hi Manfred,

Grumble.  :-) Yes, I believe you are correct.

> Setting the state to interruptible is only permitted if a full scan of
> all file descriptors happens before calling schedule(). This is
> expensive and should be avoided.
> 
> The correct fix is current->state = TASK_RUNNING just before calling
> yield() in the rebalance code.

But doesn't this have the same kind of problem?  e. g., just before
calling yield() in the rebalance code we save current->state, set it to
TASK_RUNNING, then restore current->state on return from yield().  If a
fd becomes ready after the call to yield(), and we entered
__alloc_pages() with state TASK_INTERRUPTIBLE, aren't we in exactly the
same situation as described above?

Let me think about this some more.

Thanks,
-- 
Best Regards,
Ray
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                  Ray Bryant
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