linux-kernel - Re: Do not deprecate binary semaphore or do allow mutex in software interrupt contexts

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Date:	Tue, 11 Sep 2007 15:36:28 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	Matti Linnanvuori <mattilinnanvuori@...oo.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Do not deprecate binary semaphore or do allow mutex in software
 interrupt contexts

On Tue, 11 Sep 2007 06:20:51 -0700 (PDT)
Matti Linnanvuori <mattilinnanvuori@...oo.com> wrote:

Hi,

> I thought of a scenario where it seems appropriate to use a binary
> semaphore or a mutex in a software interrupt context. If a device
> cannot interrupt when some important variable changes, it can be
> polled occasionally to update e.g. LEDs to indicate status. Such
> polling can be done most efficiently in timers that are software
> interrupts. Timers are more efficient than works because they do not
> have so much context switching overhead. If the access to the
> variables of the device must be serialized, a binary semaphore or a
> mutex is a natural choice. If user-space writing to the device is
> likely to change the status, it can make sense not to poll the status
> of the device at the same time. The timer could therefore sensibly
> call mutex_trylock.

what do you do if the trylock fails?

> Therefore, it seems wrong to me to deprecate
> binary semaphores and disallow the use of mutexes in software
> interrupt contexts.

to be honest, the scenario describe really smells of broken locking, in
fact it really sounds like it wants to use spinlocks instead 

Greetings,
   Arjan van de Ven
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