Re: [PATCH v2 1/2] Change DEFINE_SEMAPHORE() to take a number argument

From: Matthew Wilcox <willy@infradead.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: david@redhat.com, patches@lists.linux.dev,
	linux-modules@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, pmladek@suse.com,
	petr.pavlu@suse.com, prarit@redhat.com,
	torvalds@linux-foundation.org, gregkh@linuxfoundation.org,
	rafael@kernel.org, christophe.leroy@csgroup.eu,
	tglx@linutronix.de, peterz@infradead.org, song@kernel.org,
	rppt@kernel.org, dave@stgolabs.net, vbabka@suse.cz,
	mhocko@suse.com, dave.hansen@linux.intel.com,
	colin.i.king@gmail.com, jim.cromie@gmail.com,
	catalin.marinas@arm.com, jbaron@akamai.com,
	rick.p.edgecombe@intel.com
Subject: Re: [PATCH v2 1/2] Change DEFINE_SEMAPHORE() to take a number argument
Date: Fri, 7 Apr 2023 21:36:30 +0100	[thread overview]
Message-ID: <ZDB+zn3X4sac9DFU@casper.infradead.org> (raw)
In-Reply-To: <20230405203505.1343562-2-mcgrof@kernel.org>

On Wed, Apr 05, 2023 at 01:35:04PM -0700, Luis Chamberlain wrote:
> diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
> index 6694d0019a68..2d6aa3fd7861 100644
> --- a/include/linux/semaphore.h
> +++ b/include/linux/semaphore.h
> @@ -25,8 +25,15 @@ struct semaphore {
>  	.wait_list	= LIST_HEAD_INIT((name).wait_list),		\
>  }
>  
> -#define DEFINE_SEMAPHORE(name)	\
> -	struct semaphore name = __SEMAPHORE_INITIALIZER(name, 1)
> +/*
> + * There is a big difference between a binary semaphore and a mutex.
> + * You cannot call mutex_unlock() from IRQ context because it takes an
> + * internal mutex spin_lock in a non-IRQ-safe manner. Both try_lock()
> + * and unlock() can be called from IRQ context. A mutex must also be
> + * released in the same context that locked it.
> + */

I think this confuses cause and effect.  How about this:

/*
 * Binary semaphores and mutexes differ in that mutexes have an owner
 * so they cannot be used from interrupt context and cannot be passed
 * from one thread to another.  down_trylock() and up() can be called
 * from interrupt context.
 */

Or this:

/*
 * Unlike mutexes, binary semaphores do not have an owner, so up() can
 * be called in a different thread from the one which called down().
 * It is also safe to call down_trylock() and up() from interrupt
 * context.
 */

I'd like to mention completions as an alternative to semaphores, but
can't figure out a nice way to fit that in.