Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic

From: Christian Borntraeger <[email-masked]>
Date: Wed, 26 Nov 2014 17:07:13 +0100

Am 26.11.2014 um 16:47 schrieb Michael S. Tsirkin:
> On Wed, Nov 26, 2014 at 04:32:07PM +0100, David Hildenbrand wrote:
>>> 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.
>>>
>>
>> (Thanks for your resent, my first email was sent directly to you ... grml)
>>
>> 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.

Huh? With CONFIG_PREEMPT spin_lock will disable preemption. (we had preempt = server anyway).

But please: One step back. The problem is not the good path. The problem is that we lost a debugging aid for a known to be broken case. In other words: Our code had a bug. Older kernels detected that kind of bug. With your change we no longer saw the sleeping while atomic. Thats it. See my other mail.

Christian