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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-arch@vger.kernel.org, heiko.carstens@de.ibm.com,
	linux-kernel@vger.kernel.org,
	David Hildenbrand <dahi@linux.vnet.ibm.com>,
	paulus@samba.org, schwidefsky@de.ibm.com,
	akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
	mingo@kernel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic
Date: Wed, 26 Nov 2014 17:37:33 +0200	[thread overview]
Message-ID: <20141126153732.GA10568@redhat.com> (raw)
In-Reply-To: <5475F218.4050207@de.ibm.com>

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

  reply	other threads:[~2014-11-26 15:37 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-11-25 11:43 [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic David Hildenbrand
2014-11-25 11:43 ` [RFC 1/2] powerpc/fsl-pci: atomic get_user when pagefault_disabled David Hildenbrand
2015-01-30  5:15   ` [RFC,1/2] " Scott Wood
2015-01-30  7:58     ` David Hildenbrand
2014-11-25 11:43 ` [RFC 2/2] mm, sched: trigger might_sleep() in might_fault() when atomic David Hildenbrand
2014-11-26  7:02 ` [RFC 0/2] Reenable might_sleep() checks for " Michael S. Tsirkin
2014-11-26 10:05   ` David Hildenbrand
2014-11-26 15:17     ` Michael S. Tsirkin
2014-11-26 15:23       ` Michael S. Tsirkin
2014-11-26 15:32         ` David Hildenbrand
2014-11-26 15:47           ` Michael S. Tsirkin
2014-11-26 16:02             ` David Hildenbrand
2014-11-26 16:19               ` Michael S. Tsirkin
2014-11-26 16:30                 ` Christian Borntraeger
2014-11-26 16:50                   ` Michael S. Tsirkin
2014-11-26 16:07             ` Christian Borntraeger
2014-11-26 16:32               ` Michael S. Tsirkin
2014-11-26 16:51                 ` Christian Borntraeger
2014-11-26 17:04                   ` Michael S. Tsirkin
2014-11-26 17:21                     ` Michael S. Tsirkin
2014-11-27  7:09                     ` Heiko Carstens
2014-11-27  7:40                       ` Michael S. Tsirkin
2014-11-27  8:03                       ` David Hildenbrand
2014-11-27 12:04                         ` Heiko Carstens
2014-11-27 12:08                           ` David Hildenbrand
2014-11-27 15:07                           ` Thomas Gleixner
2014-11-27 15:19                             ` David Hildenbrand
2014-11-27 15:37                               ` David Laight
2014-11-27 15:45                                 ` David Hildenbrand
2014-11-27 16:27                                   ` David Laight
2014-11-27 16:49                                     ` David Hildenbrand
2014-11-27 21:52                               ` Thomas Gleixner
2014-11-28  7:34                                 ` David Hildenbrand
2014-11-26 15:30       ` Christian Borntraeger
2014-11-26 15:37         ` Michael S. Tsirkin [this message]
2014-11-26 16:02           ` Christian Borntraeger
2014-11-26 15:22     ` Michael S. Tsirkin
2014-11-27 17:10 ` [PATCH RFC " David Hildenbrand
2014-11-27 17:10   ` [PATCH RFC 1/2] preempt: track pagefault_disable() calls in the preempt counter David Hildenbrand
2014-11-27 17:10   ` [PATCH RFC 2/2] mm, sched: trigger might_sleep() in might_fault() when pagefaults are disabled David Hildenbrand
2014-11-27 17:24     ` Michael S. Tsirkin
2014-11-27 17:32       ` Michael S. Tsirkin
2014-11-27 18:08         ` David Hildenbrand
2014-11-27 18:27           ` Michael S. Tsirkin

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