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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pksow-001Bg4-Fv; Fri, 07 Apr 2023 20:36:30 +0000 Date: Fri, 7 Apr 2023 21:36:30 +0100 From: Matthew Wilcox To: Luis Chamberlain 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 Message-ID: References: <20230405203505.1343562-1-mcgrof@kernel.org> <20230405203505.1343562-2-mcgrof@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230405203505.1343562-2-mcgrof@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: zr4mrkzwthaxysef5num7wcxx61fer37 X-Rspamd-Queue-Id: 1F555A0013 X-HE-Tag: 1680899813-524308 X-HE-Meta: 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 G1x0OC/4 Wl6uLEUXTJk5cfYunItdfsMhVfSUeT7pUrcbWdzN6bRJpB+XV3W0F7hWEb6pzt+5u/zc/i8BjVs/oT/NRGs8HTRRqPXzPYPJb1jPh2mzhT199TfZTusjyOMi5oUzlVMjDSJ7L6CZPBNXe2zvrXIhYwu0pyluQNjsy7hBF43ARhB7QJ66iBgJtm4P9Deu4iC9pVHxzc9Nbsie86NRkWfcbuj5JesSsMCcdBhDPB6ZtB7/DyAt4JyimzZcK/0g/ycKITSmW0KqvqtVLG7aysKfp5M4nSA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.