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Message-id: <474627CC.10201@shaw.ca>
Date: Thu, 22 Nov 2007 19:07:24 -0600
From: Robert Hancock <hancockr@...w.ca>
To: Leon Woestenberg <leon.woestenberg@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Use of mutex in interrupt context flawed/impossible, need advice.
Leon Woestenberg 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.
Unless this update into the FPGA takes a significant amount of time, I
wouldn't bother with that complexity - just do spin_lock_irq/irqsave on
that spinlock.
Using a trylock for this rather sucks since the behavior is entirely
non-deterministic. It could take a really long time in some cases for
the trylock to ever succeed.
>
> My feeling is that we have a valid use of mutex_trylock() in
> interrupt context; "i.e. update LUT if we can do so consistently and
> in time, or not at all".
>
> I would like to know why this is not so, and if someone has a cleaner
> proposal than the "try spinlock" approach?
--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/
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