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

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Date:	Mon, 25 Mar 2013 16:21:22 -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,
	Dave Jones <davej@...hat.com>
Subject: Re: ipc,sem: sysv semaphore scalability

On 03/20/2013 03:55 PM, Rik van Riel wrote:
> Include lkml in the CC: this time... *sigh*
> ---8<---
> 
> 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,

I'm getting the following false positives from lockdep:

[   80.492995] =============================================
[   80.494052] [ INFO: possible recursive locking detected ]
[   80.494878] 3.9.0-rc4-next-20130325-sasha-00044-gcb6ef58 #315 Tainted: G        W
[   80.496228] ---------------------------------------------
[   80.497171] trinity-child9/7210 is trying to acquire lock:
[   80.497934]  (&(&sma->sem_base[i].lock)->rlock){+.+...}, at: [<ffffffff8192da37>] newary+0x1c7/0x2a0
[   80.499202]
[   80.499202] but task is already holding lock:
[   80.500031]  (&(&sma->sem_base[i].lock)->rlock){+.+...}, at: [<ffffffff8192da37>] newary+0x1c7/0x2a0
[   80.500031]
[   80.500031] other info that might help us debug this:
[   80.500031]  Possible unsafe locking scenario:
[   80.500031]
[   80.500031]        CPU0
[   80.500031]        ----
[   80.500031]   lock(&(&sma->sem_base[i].lock)->rlock);
[   80.500031]   lock(&(&sma->sem_base[i].lock)->rlock);
[   80.500031]
[   80.500031]  *** DEADLOCK ***
[   80.500031]
[   80.500031]  May be due to missing lock nesting notation
[   80.500031]
[   80.500031] 4 locks held by trinity-child9/7210:
[   80.500031]  #0:  (&ids->rw_mutex){+++++.}, at: [<ffffffff8192a422>] ipcget+0x72/0x340
[   80.500031]  #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff81929b65>] ipc_addid+0x35/0x230
[   80.500031]  #2:  (&(&new->lock)->rlock){+.+...}, at: [<ffffffff81929c00>] ipc_addid+0xd0/0x230
[   80.500031]  #3:  (&(&sma->sem_base[i].lock)->rlock){+.+...}, at: [<ffffffff8192da37>] newary+0x1c7/0x2a0
[   80.500031]
[   80.500031] stack backtrace:
[   80.500031] Pid: 7210, comm: trinity-child9 Tainted: G        W    3.9.0-rc4-next-20130325-sasha-00044-gcb6ef58 #315
[   80.500031] Call Trace:
[   80.500031]  [<ffffffff8117f65e>] __lock_acquire+0xc6e/0x1e50
[   80.500031]  [<ffffffff819ff225>] ? idr_get_empty_slot+0x255/0x3c0
[   80.500031]  [<ffffffff8117ca1e>] ? mark_held_locks+0x12e/0x150
[   80.500031]  [<ffffffff811810ba>] lock_acquire+0x1aa/0x240
[   80.500031]  [<ffffffff8192da37>] ? newary+0x1c7/0x2a0
[   80.500031]  [<ffffffff83d8768b>] _raw_spin_lock+0x3b/0x70
[   80.500031]  [<ffffffff8192da37>] ? newary+0x1c7/0x2a0
[   80.500031]  [<ffffffff8192da37>] newary+0x1c7/0x2a0
[   80.500031]  [<ffffffff8192a422>] ? ipcget+0x72/0x340
[   80.500031]  [<ffffffff8192a5d6>] ipcget+0x226/0x340
[   80.500031]  [<ffffffff81930215>] SyS_semget+0x65/0x80
[   80.500031]  [<ffffffff8192d870>] ? semctl_down.constprop.8+0x320/0x320
[   80.500031]  [<ffffffff8192c770>] ? wake_up_sem_queue_do+0xa0/0xa0
[   80.500031]  [<ffffffff8192c690>] ? SyS_msgrcv+0x20/0x20
[   80.500031]  [<ffffffff83d90898>] tracesys+0xe1/0xe6

The code is:

        for (i = 0; i < nsems; i++) {
                INIT_LIST_HEAD(&sma->sem_base[i].sem_pending);
                spin_lock_init(&sma->sem_base[i].lock);
                spin_lock(&sma->sem_base[i].lock);  <---- here
        }


Thanks,
Sasha
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