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Date: Fri, 12 Mar 2010 14:23:44 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Stefani Seibold <stefani@seibold.net>
Cc: linux-mtd@lists.infradead.org, linux-kernel <linux-kernel@vger.kernel.org>,
       David Woodhouse <dwmw2@infradead.org>,
       "Kreuzer, Michael (NSN - DE/Ulm)" <michael.kreuzer@nsn.com>
Subject: Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug
Message-Id: <20100312142344.174bd46f.akpm@linux-foundation.org>
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On Sat, 06 Mar 2010 17:48:57 +0100
Stefani Seibold <stefani@seibold.net> wrote:

> This patch fix a huge latency problem in the MTD CFI and LPDDR flash
> drivers.
> 
> The use of a memcpy() during a spinlock operation will cause very long
> thread context switch delays if the flash chip bandwidth is low and the
> data to be copied large, because a spinlock will disable preemption.
> 
> For example: A flash with 6,5 MB/s bandwidth will cause under ubifs,
> which request sometimes 128 KB (the flash erase size), a preemption
> delay of 20 milliseconds. High priority threads will not be served
> during this time, regardless whether this threads access the flash or
> not. This behavior breaks real time.
> 
> The patch change all the use of spin_lock operations for xxxx->mutex
> into mutex operations, which is exact what the name says and means. 
> 
> There is no performance regression since the mutex is normally not
> acquired.

hm, big scary patch.  Are you sure this mutex is never taken from
atomic or irq contexts?  Is it ully tested with all relevant debug options
and lockdep enabled?