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

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Message-Id: <200706231402.03617.arnd@arndb.de>
Date:	Sat, 23 Jun 2007 14:02:03 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	Florin Iucha <florin@...ha.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: "upping" a semaphore from interrupt context?

On Saturday 23 June 2007, Robert P. J. Day wrote:
> On Fri, 22 Jun 2007, Arnd Bergmann wrote:
> >
> > 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.
> 
> can you clarify this?  it sounds like you're saying that the current
> implementation of semaphores is entirely superfluous.  but surely it
> isn't possible to replace all semaphores with either mutexes or
> completions, is it?

No, not all of them, but the vast majority. There are multiple
differences, the most important one being the 'counting' in
semaphores. You can e.g. define a semaphore that can be held
by N users at the same time, but not more. In a mutex, N is
by definition 1, so only one thread can hold a mutex.

There are other subtle differences in the implementation, e.g.
you cannot mutex_trylock at interrupt time.

	Arnd <><
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