linux-kernel - Re: [PATCHv6 00/15] kill unnecessary bdi wakeups + cleanups

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Date:	Wed, 4 Aug 2010 13:34:15 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	<dedekind1@...il.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCHv6 00/15] kill unnecessary bdi wakeups + cleanups

On 2010-08-03 14:47, Jens Axboe wrote:
> On 2010-08-03 14:37, Artem Bityutskiy wrote:
>> On Tue, 2010-08-03 at 14:27 +0200, Jens Axboe wrote:
>>> On 2010-07-25 13:29, Artem Bityutskiy wrote:
>>>> Hi,
>>>>
>>>> here is v6 of the patch series which clean-ups bdi threads and substantially
>>>> lessens amount of unnecessary kernel wake-ups, which is very important on
>>>> battery-powered devices.
>>>>
>>>> This patch-set is also available at:
>>>> git://git.infradead.org/users/dedekind/misc-2.6.git flushers_v6
>>>
>>> Thanks Artem, for sticking around long enough to get this into
>>> shape. I have finally merged it.
>>
>> Thanks, but
>>
>>>> 1. Use 'spin_lock_bh' for the 'bdi->wb_lock' (changed patch N12)
>>>
>>> I'd rather not, question is how to avoid it. Either just wakeup the
>>> default thread, or punt the lock-and-check bdi->wb.task to a thread.
>>
>> you merged this change, do you want me to send a separate patch which
>> undo the 'spin_lock_bh' change? I'll think about how to avoid this and
>> come back.
> 
> Yes, it's not a huge thing, but it would be nice to get rid of. So I
> figured it was better to merge it and not have you respin the series yet
> again.

There is a spinlock bug in the current code, you nest _bh locks on lock
but not always on unlock. I fixed it up as per the below:

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 0b8ee66..08d3575 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -415,7 +415,8 @@ static int bdi_forker_thread(void *ptr)
 				break;
 			}
 
-			spin_lock_bh(&bdi->wb_lock);
+			spin_lock(&bdi->wb_lock);
+
 			/*
 			 * If there is no work to do and the bdi thread was
 			 * inactive long enough - kill it. The wb_lock is taken
@@ -432,7 +433,7 @@ static int bdi_forker_thread(void *ptr)
 				action = KILL_THREAD;
 				break;
 			}
-			spin_unlock_bh(&bdi->wb_lock);
+			spin_unlock(&bdi->wb_lock);
 		}
 		spin_unlock_bh(&bdi_lock);
 

-- 
Jens Axboe


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