Re: linux-next 20170519 - semaphores broken

[Manfred Spraul]

Hi valdis,

On 05/20/2017 10:18 PM, valdis.kletnieks@xxxxxx wrote:
> Seeing problems with programs that use semaphores.  The one
> that I'm getting bit by is jackd.  strace says:
>
> getuid()                                = 967
> semget(0x282929, 0, 000)                = 229376
> semop(229376, [{0, -1, SEM_UNDO}], 1)   = -1 EIDRM (Identifier removed)
> write(2, "JACK semaphore error: semop (Ide"..., 49JACK semaphore error: semop (Identifier removed)
> ) = 49
>
> Bisects down to this commit, and reverting it from 20170519 makes things work
> again.  No idea why this causes indigestion, there's probably something subtly
> wrong here....
Duh, thanks.
There was another report that got a 99% improvement, which I did not 
understand.

Probable root cause:
>         sma = container_of(ipc_rcu_alloc(size), struct sem_array, 
> sem_perm);
>         if (!sma)
>                 return -ENOMEM;
>
>         memset(sma, 0, size);
sma->sem_perm.refcount was initialized by ipc_rcu_alloc.
And due to the SEM_UNDO, the refcount is relevant.

Thanks for bisecting it, I'll update the patch.

--
    Manfred