Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 26 Nov 2014 17:47:17 +0200

On Wed, Nov 26, 2014 at 04:32:07PM +0100, David Hildenbrand wrote:
> > On Wed, Nov 26, 2014 at 05:17:29PM +0200, Michael S. Tsirkin wrote:
> > > On Wed, Nov 26, 2014 at 11:05:04AM +0100, David Hildenbrand wrote:
> > > > > What's the path you are trying to debug?
> > > > 
> > > > Well, we had a problem where we held a spin_lock and called
> > > > copy_(from|to)_user(). We experienced very random deadlocks that took some guy
> > > > almost a week to debug. The simple might_sleep() check would have showed this
> > > > error immediately.
> > > 
> > > This must have been a very old kernel.
> > > A modern kernel will return an error from copy_to_user.
> > > Which is really the point of the patch you are trying to revert.
> > 
> > That's assuming you disabled preemption. If you didn't, and take
> > a spinlock, you have deadlocks even without userspace access.
> > 
> 
> (Thanks for your resent, my first email was sent directly to you ... grml)
> 
> This is what happened on our side (very recent kernel):
> 
> spin_lock(&lock)
> copy_to_user(...)
> spin_unlock(&lock)

That's a deadlock even without copy_to_user - it's
enough for the thread to be preempted and another one
to try taking the lock.

> 1. s390 locks/unlocks a spin lock with a compare and swap, using the _cpu id_
>    as "old value"
> 2. we slept during copy_to_user()
> 3. the thread got scheduled onto another cpu
> 4. spin_unlock failed as the _cpu id_ didn't match (another cpu that locked
>    the spinlock tried to unlocked it).
> 5. lock remained locked -> deadlock
> 
> Christian came up with the following explanation:
> Without preemption, spin_lock() will not touch the preempt counter.
> disable_pfault() will always touch it.
> 
> Therefore, with preemption disabled, copy_to_user() has no idea that it is
> running in atomic context - and will therefore try to sleep.
>
> So copy_to_user() will on s390:
> 1. run "as atomic" while spin_lock() with preemption enabled.
> 2. run "as not atomic" while spin_lock() with preemption disabled.
> 3.  run "as atomic" while pagefault_disabled() with preemption enabled or
> disabled.
> 4. run "as not atomic" when really not atomic.
> 
> And exactly nr 2. is the thing that produced the deadlock in our scenario and
> the reason why I want a might_sleep() :)

IMHO it's not copy to user that causes the problem.
It's the misuse of spinlocks with preemption on.

So might_sleep would make you think copy_to_user is
the problem, and e.g. let you paper over it by
moving copy_to_user out.

Enable lock prover and you will see what the real
issue is, which is you didn't disable preempt.
and if you did, copy_to_user would be okay.

-- 
MST