linux-kernel - Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll

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Date:   Tue, 08 Jan 2019 13:42:25 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Davidlohr Bueso <dbueso@...e.de>, Jason Baron <jbaron@...mai.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is
 always called with irqs off

On 2019-01-08 11:01, Roman Penyaev wrote:
> That was wrong assumption that all drivers disable irqs before waking 
> up
> a wait queue.  Even assert line is removed the whole logic stays 
> correct:
> epoll always locks rwlock with irqs disabled and by itself does not 
> call
> from interrupts, thus it is up to driver how to call wake_up_locked(),
> because if driver does not handle any interrupts (like fuse in the the
> report) of course it is safe on its side to take a simple spin_lock.

This is wrong and can lead to dead lock: we always call read_lock(), 
caller
can call us with irqs enabled.  Another driver, which also calls
ep_poll_callback(), can be called from interrupt context (irqs disabled)
thus it can interrupt the one who does not disable irqs.  Even we take
a read_lock() (which should be fine to interrupt), write_lock(), which
comes in the middle, can cause a dead lock:

#CPU0                      #CPU1

task:                      task:                       irq:

                            spin_lock(&wq1->lock);
                            ep_poll_callback():
                            read_lock(&ep->lock)
                            ....
write_lock_irq(&ep->lock)  ....
   #waits reads             ....          >>>>>>>>>>>>>>  IRQ CPU1
                                                       
spin_lock_irqsave(&wq2->lock)
                                                       
ep_poll_callback():
                                                       
read_lock(&ep->lock);
                                                       # to avoid write 
starve should
                                                       # wait writer to 
finish, thus
                                                       # dead lock


What we can do:

a) disable irqs if we are not in interrupt.
b) revert the patch completely.

David, is it really crucial in terms of performance to avoid double
local_irq_save() on Xen on this ep_poll_callback() hot path?

For example why not to do the following:

   if (!in_interrupt())
        local_irq_save(flags);
   read_lock(ep->lock);

with huge comment explaining performance number.

Or just give up and simply revert the original patch completely
and always call read_lock_irqsave().

--
Roman