Re: [patch 0/3] futex/rtmutex: Fix issues exposed by trinity

[Carlos O'Donell]

On 05/14/2014 07:11 PM, Thomas Gleixner wrote:
> On Wed, 14 May 2014, Carlos O'Donell wrote:
>> On 05/14/2014 05:22 AM, Peter Zijlstra wrote:
>>>>> I believe the thinking goes that if we get to here, then the lock is in an
>>>>> inconsistent state (between kernel and userspace). I don't have an answer for
>>>>> why pausing forever would be preferable to returning an error however...
>>>>
>>>> What error would we return?
>>>
>>> EDEADLK is a valid user return for pthread_mutex_lock() as per:
>>>
>>>   http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html
>>
>> How is that correct? It isn't a deadlock we've detected but inconsistent
>> state between glibc and the kernel. In this case glibc should assert.
>> Delaying indefinitely with pause() never seems correct (despite that being
>> what we do today).
> 
> If there is inconsistent state detected then the kernel will return
> -EPERM or -EINVAL. So lets put inconsistent state aside.

OK.
 
> In glibc you only can detect the simple AA dead lock, i.e lock owner
> tries to lock the lock it owns again. Trivial, right ?

Agreed.

> But glibc has no idea which lock chains are involved and might lead to
> a dead lock caused by nested locking, simplest and most popular being
> ABBA.

OK.

> The kernel can (if the implementation is fixed, patch is available
> already) very well detect ABBA and even more complex nested lock
> deadlocks. So it rightfully returns -EDEADLK and that is completely
> correct versus the spec and the call site can do something about it.

OK.

> And that's not different from the glibc detected AA deadlock at
> all. It's just detected by a different mechanism.

OK.

> On kernel side we currently provide this service only for the PI
> futexes because we have a kernel side state representation as long as
> the user space state is not corrupted.

OK.

> Back then when it was implemented the dead lock detection actually
> worked and was agreed on by both sides - kernel and glibc - to be
> usefull and essential to the whole endavour.

I agree that ignoring the situation of corrupted or inconsistent
state we should be returning EDEADLK to userspace.

We'll cleanup glibc.

Cheers,
Carlos.

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