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

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Message-ID: <20141126153732.GA10568@redhat.com>
Date:	Wed, 26 Nov 2014 17:37:33 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	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, mingo@...nel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

On Wed, Nov 26, 2014 at 04:30:32PM +0100, Christian Borntraeger wrote:
> Am 26.11.2014 um 16:17 schrieb Michael S. Tsirkin:
> > 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.
> 
> I disagree. copy_to_user will not return while holding a spinlock, because it does not know! How should it?
> See: spin_lock will call preempt_disable, but thats a no-op for a non-preempt kernel. So the mere fact that we hold a spin_lock is not known by any user access function. (or others). No?
> 
> Christian
> 
> 

Well might_sleep() merely checks preempt count and irqs_disabled too.
If you want debugging things to trigger, you need to enable
a bunch of config options.  That's not new.


-- 
MST
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