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 17:23:34 +0200

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.

-- 
MST