Date: Thu, 22 Nov 2007 09:17:32 -0800
From: Arjan van de Ven <arjan@infradead.org>
To: "Leon Woestenberg" <leon.woestenberg@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Use of mutex in interrupt context flawed/impossible, need
 advice.
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On Thu, 22 Nov 2007 17:02:44 +0100
"Leon Woestenberg" <leon.woestenberg@gmail.com> wrote:

> Hello,
> 
> 
> I'm converting an out-of-tree (*1) driver from binary semaphore to
> mutex.
> 
> Userspace updates a look-up-table using write(). The driver tries to
> write this LUT to the FPGA in the (video frame) interrupt handler. It
> is important that the LUT is consistent and thus changed atomically.
> Note that it is not important that the LUT is updated each interrupt.
> 
> The current approach is to try-down()ing a binary semaphore in
> interrupt context, and write the LUT to the FPGA if the semaphore was
> down()ed, do nothing else.
> The write() down()s the semaphore as well before updating the
> in-driver-copy of the LUT, then up()s it again.
> 
> I understand this design is not clean (*2), and not even possible with
> mutexes, as mutex_trylock() is not interrupt safe.
> 
> My current approach would be to have userspace write into a shadow
> copy, and use a spinlock to update the live copy. The interrupt then
> would try a spinlock.
>

I suspect you need to copy the userspace data anyway, so I don't see
why the spinlock approach would be wrong; if the update itself is short
and non-sleeping, it's even better than a mutex or semaphore.


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