On 01/04/2016 02:02 PM, Davidlohr Bueso wrote:
> On Sat, 02 Jan 2016, Manfred Spraul wrote:
>
>> Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a 
>> race:
>>
>> sem_lock has a fast path that allows parallel simple operations.
>> There are two reasons why a simple operation cannot run in parallel:
>> - a non-simple operations is ongoing (sma->sem_perm.lock held)
>> - a complex operation is sleeping (sma->complex_count != 0)
>>
>> As both facts are stored independently, a thread can bypass the current
>> checks by sleeping in the right positions. See below for more details
>> (or kernel bugzilla 105651).
>>
>> The patch fixes that by creating one variable (complex_mode)
>> that tracks both reasons why parallel operations are not possible.
>>
>> The patch also updates stale documentation regarding the locking.
>>
>> With regards to stable kernels:
>> The patch is required for all kernels that include the commit 
>> 6d07b68ce16a
>> ("ipc/sem.c: optimize sem_lock()") (3.10?)
>>
>> The alternative is to revert the patch that introduced the race.
>
> I am just now catching up with this, but a quick thought is that we 
> probably
> want to keep 6d07b68ce16a as waiting on unlocking all sem->lock should be
> much worse for performance than keeping track of the complex 'mode'.
Keeping track is simple - and the fast path gets simpler compared to 
ncurrent code.
> Specially
> if we have a large array.
Yes, I would prefer my patch as a backport.
But if someone has objections, then a revert would be an alternative.

>
> Also, any idea what workload exposed this race? Anyway, will take a 
> closer look
> at the patch/issue.
It was found by a theoretical review:
The sem_lock code was used as an exercise at University Bremen.

--
     Manfred
P.S.: If we replace the "bool" with an "int", we could even introduce a 
hysteresis, to further reduce the amount of array scans.