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

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Date:	Thu, 11 Apr 2013 08:57:01 -0700
From:	Eric Dumazet <erdnetdev@...il.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Bart Van Assche <bvanassche@....org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] spinlock: split out debugging check from
 spin_lock_mutex

On Thu, 2013-04-11 at 11:18 -0400, Neil Horman wrote:
> Bart, this patch should fix your problem.  Could you please test it and confirm?
> 
> Bart Van Assche recently reported a warning to me:
> 
> <IRQ>  [<ffffffff8103d79f>] warn_slowpath_common+0x7f/0xc0
>  [<ffffffff8103d7fa>] warn_slowpath_null+0x1a/0x20
>  [<ffffffff814761dd>] mutex_trylock+0x16d/0x180
>  [<ffffffff813968c9>] netpoll_poll_dev+0x49/0xc30
>  [<ffffffff8136a2d2>] ? __alloc_skb+0x82/0x2a0
>  [<ffffffff81397715>] netpoll_send_skb_on_dev+0x265/0x410
>  [<ffffffff81397c5a>] netpoll_send_udp+0x28a/0x3a0
>  [<ffffffffa0541843>] ? write_msg+0x53/0x110 [netconsole]
>  [<ffffffffa05418bf>] write_msg+0xcf/0x110 [netconsole]
>  [<ffffffff8103eba1>] call_console_drivers.constprop.17+0xa1/0x1c0
>  [<ffffffff8103fb76>] console_unlock+0x2d6/0x450
>  [<ffffffff8104011e>] vprintk_emit+0x1ee/0x510
>  [<ffffffff8146f9f6>] printk+0x4d/0x4f
>  [<ffffffffa0004f1d>] scsi_print_command+0x7d/0xe0 [scsi_mod]
> 
> This resulted from my commit ca99ca14c which introduced a mutex_trylock
> operation in a path that could execute in interrupt context.  When mutex
> debugging is enabled, the above warns the user when we are in fact exectuting in
> interrupt context.
> 
> I think this is a false positive however.  The check is intended to catch users
> who might be issuing sleeping calls in irq context, but the use of mutex_trylock
> here is guaranteed not to sleep.
> 
> We could fix this by replacing the DEBUG_LOCK_WARN_ON check in spin_lock_mutex
> with a __might_sleep call in the appropriate parent mutex operations, but for
> the sake of effiency (which It seems is why the check was put in the spin lock
> code only when debug is enabled), lets split the spin_lock_mutex call into two
> components, where the outer component does the debug checking.  Then
> mutex_trylock can just call the inner part as its callable from irq context
> safely.
> 
> Signed-off-by: Neil Horman <nhorman@...driver.com>
> Reported-by: Bart Van Assche <bvanassche@....org>
> CC: Bart Van Assche <bvanassche@....org>
> CC: David Miller <davem@...emloft.net>
> CC: netdev@...r.kernel.org

Please post your patch to lkml ?

Thanks !


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