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

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Message-ID: <5151DBD3.6080201@oracle.com>
Date:	Tue, 26 Mar 2013 13:33:07 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Rik van Riel <riel@...riel.com>
CC:	torvalds@...ux-foundation.org, davidlohr.bueso@...com,
	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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: ipc,sem: sysv semaphore scalability

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?


Thanks,
Sasha
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