Re: ipc,sem: sysv semaphore scalability

[Davidlohr Bueso]

On Fri, 2013-03-29 at 19:09 -0700, Linus Torvalds wrote:
> On Fri, Mar 29, 2013 at 6:36 PM, Emmanuel Benisty <benisty.e@xxxxxxxxx> wrote:
> >
> > I had to slightly modify the patch since it wouldn't match the changes
> > introduced by 7-7-ipc-sem-fine-grained-locking-for-semtimedop.patch,
> > hope that was the right thing to do. So, what I tried was: original 7
> > patches + the one liner + your patch blindly modified by me on the top
> > of 3.9-rc4 and I'm still having twilight zone issues.
> 
> Ok, please send your patch so that I can double-check what you did,
> but it was simple enough that you probably did the right thing.
> 
> Sad. Your case definitely looks like a double rcu-free, as shown by
> the fact that when you enabled SLUB debugging the oops happened with
> the use-after-free pattern (it's __rcu_reclaim() doing the
> "head->func(head);" thing, and "func" is 0x6b6b6b6b6b6b6b6b, so "head"
> has already been free'd once).
> 
> So ipc_rcu_putref() and a refcounting error looked very promising.as a
> potential explanation.
> 
> The 'un' undo structure is also free'd with rcu, but the locking
> around that seems much more robust. The undo entry is on two lists
> (sma->list_id, under sma->sem_perm.lock and ulp->list_proc, under
> ulp->lock). But those locks are actually tested with
> assert_spin_locked() in all the relevant places, and the code actually
> looks sane. So I had high hopes for ipc_rcu_putref()...
> 
> Hmm. Except for exit_sem() that does odd things. You have preemption
> enabled, don't you? exit_sem() does a lookup of the first list_proc
> entry under tcy_read_lock to lookup un->semid, and then it drops the
> rcu read lock. At which point "un" is no longer reliable, I think. But
> then it still uses "un->semid", rather than the stable value it looked
> up under the rcu read lock. Which looks bogus.
> 
> So I'd like you to test a few more things:
> 
>  (a) In exit_sem(), can you change the
> 
>          sma = sem_lock_check(tsk->nsproxy->ipc_ns, un->semid);
> 
>      to use just "semid" rather than "un->semid", because I don't
> think "un" is stable here.

Well that's not really the case in the new code. We don't drop the rcu
read lock until the end of the loop, in sem_unlock(). However, I just
noticed that we're checking sma for error after trying to acquire
sma->sem_perm.lock:

		sma = sem_obtain_object_check(tsk->nsproxy->ipc_ns, un->semid);
		sem_lock(sma, NULL, -1);

		/* exit_sem raced with IPC_RMID, nothing to do */
		if (IS_ERR(sma))
			continue;

The IS_ERR(sma) check should be right after the sem_obtain_object_check() call instead.


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