netdev - Re: [PATCH RFC] spinlock: split out debugging check from spin_lock

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Date:	Mon, 29 Apr 2013 14:13:22 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	bvba Bart Van Assche <info@...tvanassche.be>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH RFC] spinlock: split out debugging check from
 spin_lock_mutex

On Sat, Apr 27, 2013 at 08:53:54PM +0200, bvba Bart Van Assche wrote:
> On 04/23/13 19:50, Neil Horman wrote:
> > On Tue, Apr 23, 2013 at 01:33:15PM -0400, David Miller wrote:
> >> From: Neil Horman <nhorman@...driver.com>
> >> Date: Tue, 23 Apr 2013 09:44:43 -0400
> >>
> >>> Dave, how do you feel about it?  I'm comfortable with the wait queue change I've
> >>> proposed, but I've not received any reports of actual netpoll deadlocks (i.e.
> >>> the mutex solution is reporting a warning, but no actual problems).  So I think
> >>> its safe to wait a bit longer, unless you just want this squared away now.
> >>
> >> If it's just a warning and people aren't actually hitting the
> >> potential deadlock, it can wait.
> >>
> > Copy that. Bart, I'll wait till you get back then.
> 
> (Just arrived home)
> 
> Sorry Neil, but I can still trigger the CPU stuck messages with kernel v3.9-rc8-24-gd7d7271:
> 
> kernel: BUG: soft lockup - CPU#0 stuck for 22s! [rs:main Q:Reg:601]
> kernel: irq event stamp: 1999192
> kernel: hardirqs last  enabled at (1999191): [<ffffffff8103e89d>] console_unlock+0x41d/0x450
> kernel: hardirqs last disabled at (1999192): [<ffffffff8143e96a>] apic_timer_interrupt+0x6a/0x80
> kernel: softirqs last  enabled at (1999188): [<ffffffff81044e26>] __do_softirq+0x196/0x280
> kernel: softirqs last disabled at (1999181): [<ffffffff810450c5>] irq_exit+0xb5/0xc0
> kernel: CPU 0 
> kernel: Pid: 601, comm: rs:main Q:Reg Tainted: G           O 3.9.0-rc8-debug+ #1 System manufacturer P5Q DELUXE/P5Q DELUXE
> kernel: RIP: 0010:[<ffffffff8103e8a0>]  [<ffffffff8103e8a0>] console_unlock+0x420/0x450
> kernel: Call Trace:
> kernel: [<ffffffff812bbf37>] do_con_write.part.19+0x887/0x2040
> kernel: [<ffffffff812a52b7>] ? process_output+0x37/0x70
> kernel: [<ffffffff814316fc>] ? mutex_lock_nested+0x28c/0x350
> kernel: [<ffffffff812a52b7>] ? process_output+0x37/0x70
> kernel: [<ffffffff812bd764>] con_write+0x34/0x50
> kernel: [<ffffffff812a51e9>] do_output_char+0x179/0x210
> kernel: [<ffffffff812a52cd>] process_output+0x4d/0x70
> kernel: [<ffffffff812a59d0>] n_tty_write+0x210/0x480
> kernel: [<ffffffff81072710>] ? try_to_wake_up+0x2e0/0x2e0
> kernel: [<ffffffff812a2839>] tty_write+0x159/0x300
> kernel: [<ffffffff8109326f>] ? lock_release_holdtime.part.22+0xf/0x180
> kernel: [<ffffffff812a57c0>] ? n_tty_poll+0x1c0/0x1c0
> kernel: [<ffffffff81151a3b>] vfs_write+0xab/0x170
> kernel: [<ffffffff81151ea5>] sys_write+0x55/0xa0
> kernel: [<ffffffff8143dd82>] system_call_fastpath+0x16/0x1b
> 
> Bart.
> 

So, looking at this, I'm not 100% sure whats going on.  console_unlock has the
ability to detect the need for rescheduling when we call
raw_spin_unlock_irqrestore at the bottom of the function, so there should be no
reason that we don't give the per-cpu watchdog task a chance to run.  About the
only case I see in which we might be in trouble would be if somehow got into an
infinite loop if every message in the log buffer had LOG_NOCONS set, in which
case we would spin in this loop without ever scheduling or re-enabling
interrupts.  But for that to happen I think you would have to be flooding the
log with messages that were bigger than the default message buffer size (about
1000 bytes without a newline).  You're not doing that are you?

Regardless, this is a different problem from the one we were origionally
addressing, we should probably handle it in a different thread.  

On the up side, I did discover while looking at this that semaphores, unlike
mutexes are quite liberal in what contexts they can be used in (i.e. irq
contexts are perfectly legitimate places to call up and down_trylock).  I'll
respin my patch to use that, as its much simpler than a wait queue

Best
Neil

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