Re: PROBLEM: Bug in __pollwait() can cause select() and poll() to hang in 2.4.21

[Andi Kleen <ak@suse.de>]

Ray Bryant <raybry@sgi.com> writes:
> 
>      select() and poll() call a common routine: __pollwait().  On the
> first call to __pollwait(), it calls __get_free_page(GFP_KERNEL) to
> allocate a table to hold wait queues.  In the natural course of things,
> this calls into __alloc_pages().  In low memory situations, the process
> can then end up in the rebalance code at the bottom of __alloc_pages()
> where there is a call to yield().  If the process makes this call, this
> is a bad thing [tm], since the process state at that point is
> TASK_INTERRUPTIBLE.  There is no wait queue yet for the process (that is
> done later in __pollwait()) and no schedule timeout event has yet been
> created (that is done later in select()) so the process will never
> return from the call to yield().

Nasty bug. How about adding a BUG() for current->state != TASK_RUNNING at 
the beginning of __alloc_pages unless GFP_ATOMIC is set?

-Andi