From: Christian Borntraeger <borntraeger@de.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: linux-arch@vger.kernel.org, heiko.carstens@de.ibm.com,
linux-kernel@vger.kernel.org, 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 17:30:35 +0100 [thread overview]
Message-ID: <5476002B.30900@de.ibm.com> (raw)
In-Reply-To: <20141126161947.GA10850@redhat.com>
Am 26.11.2014 um 17:19 schrieb Michael S. Tsirkin:
> On Wed, Nov 26, 2014 at 05:02:23PM +0100, David Hildenbrand wrote:
>>>> This is what happened on our side (very recent kernel):
>>>>
>>>> spin_lock(&lock)
>>>> copy_to_user(...)
>>>> spin_unlock(&lock)
>>>
>>> That's a deadlock even without copy_to_user - it's
>>> enough for the thread to be preempted and another one
>>> to try taking the lock.
>>>
>>>
>>>> 1. s390 locks/unlocks a spin lock with a compare and swap, using the _cpu id_
>>>> as "old value"
>>>> 2. we slept during copy_to_user()
>>>> 3. the thread got scheduled onto another cpu
>>>> 4. spin_unlock failed as the _cpu id_ didn't match (another cpu that locked
>>>> the spinlock tried to unlocked it).
>>>> 5. lock remained locked -> deadlock
>>>>
>>>> Christian came up with the following explanation:
>>>> Without preemption, spin_lock() will not touch the preempt counter.
>>>> disable_pfault() will always touch it.
>>>>
>>>> Therefore, with preemption disabled, copy_to_user() has no idea that it is
>>>> running in atomic context - and will therefore try to sleep.
>>>>
>>>> So copy_to_user() will on s390:
>>>> 1. run "as atomic" while spin_lock() with preemption enabled.
>>>> 2. run "as not atomic" while spin_lock() with preemption disabled.
>>>> 3. run "as atomic" while pagefault_disabled() with preemption enabled or
>>>> disabled.
>>>> 4. run "as not atomic" when really not atomic.
>>
>> should have been more clear at that point:
>> preemption enabled == kernel compiled with preemption support
>> preemption disabled == kernel compiled without preemption support
>>
>>>>
>>>> And exactly nr 2. is the thing that produced the deadlock in our scenario and
>>>> the reason why I want a might_sleep() :)
>>>
>>> IMHO it's not copy to user that causes the problem.
>>> It's the misuse of spinlocks with preemption on.
>>
>> As I said, preemption was off.
>
> off -> disabled at compile time?
>
> But the code is broken for people that do enable it.
[...]
> You should normally disable preemption if you take
> spinlocks.
Your are telling that any sequence of
spin_lock
...
spin_unlock
is broken with CONFIG_PREEMPT?
Michael, that is bullshit. spin_lock will take care of CONFIG_PREEMPT just fine.
Only sequences like
spin_lock
...
schedule
...
spin_unlock
are broken.
But as I said. That is not the problem that we are discussing here.
Christian
next prev parent reply other threads:[~2014-11-26 16:30 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 [this message]
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
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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