Date: Wed, 26 Nov 2014 17:23:34 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, benh@kernel.crashing.org,
        paulus@samba.org, akpm@linux-foundation.org, heiko.carstens@de.ibm.com,
        schwidefsky@de.ibm.com, borntraeger@de.ibm.com, mingo@kernel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic
Message-ID: <20141126152334.GA9648@redhat.com>
References: <1416915806-24757-1-git-send-email-dahi@linux.vnet.ibm.com>
 <20141126070258.GA25523@redhat.com>
 <20141126110504.511b733a@thinkpad-w530>
 <20141126151729.GB9612@redhat.com>
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In-Reply-To: <20141126151729.GB9612@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org

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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