linux-kernel - Re: "upping" a semaphore from interrupt context?

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Message-Id: <200706221934.18436.arnd@arndb.de>
Date:	Fri, 22 Jun 2007 19:34:18 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Florin Iucha <florin@...ha.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: "upping" a semaphore from interrupt context?

On Friday 22 June 2007, Florin Iucha wrote:
> I am writing a USB driver for some custom hardware, and I need to
> synchronize between the user-space and the USB subsystem.  Can I
> create a semaphore and "down" it in the reader then "up" it in the
> completion handler?

yes, but you should not. The use of semaphores is not recommended
for new code, it should be replaced with either a mutex or a
completion.

> I know the completion handler runs in interrupt context so you are not
> allowed to acquire any semaphores: but can you release them?  Will the
> waiting tasks wake up after the handler and its caller returned - IOW
> will the waking up task run in interrupt context as well?

What you should use is a 'struct completion', which you can wait for
in process context and complete from interrupt context.

	Arnd <><
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