linux-kernel - Re: [RFC 0/2] Reenable might_sleep() checks for might

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Message-ID: <20141126152334.GA9648@redhat.com>
Date:	Wed, 26 Nov 2014 17:23:34 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	paulus@...ba.org, akpm@...ux-foundation.org,
	heiko.carstens@...ibm.com, schwidefsky@...ibm.com,
	borntraeger@...ibm.com, mingo@...nel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

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
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