linux-kernel - Re: ipc,sem: sysv semaphore scalability

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Date:	Tue, 26 Mar 2013 10:51:37 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Rik van Riel <riel@...riel.com>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	hhuang@...hat.com, jason.low2@...com, walken@...gle.com,
	lwoodman@...hat.com, chegu_vinod@...com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: ipc,sem: sysv semaphore scalability

On Tue, 2013-03-26 at 13:33 -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.

This logic was from the original code, which I also found to be quite
confusing.

> 
> 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?
> 

Not necessarily, we do release everything at the end of the function: 

out_unlock_free:
	sem_unlock(sma, locknum);

Thanks,
Davidlohr

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