From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752391AbeDFVBy (ORCPT ); Fri, 6 Apr 2018 17:01:54 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:42762 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751410AbeDFVBx (ORCPT ); Fri, 6 Apr 2018 17:01:53 -0400 Subject: Re: [PATCH v4 1/3] locking: Document the semantics of spin_is_locked() To: Andrea Parri , paulmck@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, Alan Stern , David Howells , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , Jade Alglave , Luc Maranget , Akira Yokosawa , Ingo Molnar References: <1522600912-6208-2-git-send-email-andrea.parri@amarulasolutions.com> <1523044060-5568-1-git-send-email-andrea.parri@amarulasolutions.com> From: Randy Dunlap Message-ID: Date: Fri, 6 Apr 2018 14:01:41 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <1523044060-5568-1-git-send-email-andrea.parri@amarulasolutions.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/06/2018 12:47 PM, Andrea Parri wrote: > There appeared to be a certain, recurrent uncertainty concerning the > semantics of spin_is_locked(), likely a consequence of the fact that > this semantics remains undocumented or that it has been historically > linked to the (likewise unclear) semantics of spin_unlock_wait(). > > A recent auditing [1] of the callers of the primitive confirmed that > none of them are relying on particular ordering guarantees; document > this semantics by adding a docbook header to spin_is_locked(). Also, > describe behaviors specific to certain CONFIG_SMP=n builds. > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > https://marc.info/?l=linux-kernel&m=152042843808540&w=2 > https://marc.info/?l=linux-kernel&m=152043346110262&w=2 > > Co-Developed-by: Andrea Parri > Co-Developed-by: Alan Stern > Co-Developed-by: David Howells > Signed-off-by: Andrea Parri > Signed-off-by: Alan Stern > Signed-off-by: David Howells > Cc: Will Deacon > Cc: Peter Zijlstra > Cc: Boqun Feng > Cc: Nicholas Piggin > Cc: Jade Alglave > Cc: Luc Maranget > Cc: "Paul E. McKenney" > Cc: Akira Yokosawa > Cc: Ingo Molnar > --- > include/linux/spinlock.h | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h > index 4894d322d2584..1e8a464358384 100644 > --- a/include/linux/spinlock.h > +++ b/include/linux/spinlock.h > @@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock) > raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ > }) > > +/** > + * spin_is_locked() - Check whether a spinlock is locked. > + * @lock: Pointer to the spinlock. > + * > + * This function is NOT required to provide any memory ordering > + * guarantees; it could be used for debugging purposes or, when > + * additional synchronization is needed, accompanied with other > + * constructs (memory barriers) enforcing the synchronization. > + * > + * Returns: 1 if @lock is locked, 0 otherwise. Sorry, minor nit: s/Returns:/Return:/ (according to kernel-doc.rst) although I agree that "Returns:" is better. [I should have changed that years ago.] > + * > + * Note that the function only tells you that the spinlock is > + * seen to be locked, not that it is locked on your CPU. > + * > + * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n, > + * the return value is always 0 (see include/linux/spinlock_up.h). > + * Therefore you should not rely heavily on the return value. > + */ > static __always_inline int spin_is_locked(spinlock_t *lock) > { > return raw_spin_is_locked(&lock->rlock); > -- ~Randy