On Tue, Mar 26, 2013 at 01:33:07PM -0400, Sasha Levin wrote:
> On 03/20/2013 03:55 PM, Rik van Riel wrote:
> > This series makes the sysv semaphore code more scalable,
> > by reducing the time the semaphore lock is held, and making
> > the locking more scalable for semaphore arrays with multiple
> > semaphores.
>
> Hi Rik,
>
> Another issue that came up is:
>
> [ 96.347341] ================================================
> [ 96.348085] [ BUG: lock held when returning to user space! ]
> [ 96.348834] 3.9.0-rc4-next-20130326-sasha-00011-gbcb2313 #318 Tainted: G
> W
> [ 96.360300] ------------------------------------------------
> [ 96.361084] trinity-child9/7583 is leaving the kernel with locks still
> held!
> [ 96.362019] 1 lock held by trinity-child9/7583:
> [ 96.362610] #0: (rcu_read_lock){.+.+..}, at: [<ffffffff8192eafb>]
> SYSC_semtimedop+0x1fb/0xec0
>
> It seems that we can leave semtimedop without releasing the rcu read lock.
>
> I'm a bit confused by what's going on in semtimedop with regards to rcu read
> lock, it
> seems that this behaviour is actually intentional?
>
> rcu_read_lock();
> sma = sem_obtain_object_check(ns, semid);
> if (IS_ERR(sma)) {
> if (un)
> rcu_read_unlock();
> error = PTR_ERR(sma);
> goto out_free;
> }
>
> When I've looked at that it seems that not releasing the read lock was (very)
> intentional.
>
> After that, the only code path that would release the lock starts with:
>
> if (un) {
> ...
>
> So we won't release the lock at all if un is NULL?
Intentions notwithstanding, it is absolutely required to exit any and
all RCU read-side critical sections prior to going into user mode.
I suggest removing the "if (un)".
Thanx, Paul
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