From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752246AbeDFV6f (ORCPT ); Fri, 6 Apr 2018 17:58:35 -0400 Received: from mail-wr0-f193.google.com ([209.85.128.193]:43214 "EHLO mail-wr0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751965AbeDFV6e (ORCPT ); Fri, 6 Apr 2018 17:58:34 -0400 X-Google-Smtp-Source: AIpwx4+DAy32pEZb/Q53EIyb7CZ51Jf4e9Z/KruxMcv06Au34r0ig/WoNBFvJYBv+UyYyvEyTBXLvA== Date: Fri, 6 Apr 2018 23:58:25 +0200 From: Andrea Parri To: Randy Dunlap Cc: paulmck@linux.vnet.ibm.com, 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 Subject: Re: [PATCH v4 1/3] locking: Document the semantics of spin_is_locked() Message-ID: <20180406215825.GA3646@andrea> References: <1522600912-6208-2-git-send-email-andrea.parri@amarulasolutions.com> <1523044060-5568-1-git-send-email-andrea.parri@amarulasolutions.com> <20180406210741.GF3948@linux.vnet.ibm.com> <30d15756-1cb4-7123-b4d9-ce2e66861df3@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30d15756-1cb4-7123-b4d9-ce2e66861df3@infradead.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 06, 2018 at 02:08:16PM -0700, Randy Dunlap wrote: > On 04/06/2018 02:07 PM, Paul E. McKenney wrote: > > On Fri, Apr 06, 2018 at 02:01:41PM -0700, Randy Dunlap wrote: > >> 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.] > > > > Agreed, English grammar and templates often seem to conflict. > > > > So should we change this comment, or are you instead proposing to add > > "Returns:" as valid kernel-doc? > > Please change this patch to current doc syntax. > Any changes to kernel-doc syntax would come later. Paul: I understand that you're going to do this change "in place"; please let me know if I'm wrong/if you need a new submission. Thanks, Andrea > > Thanks. > > > Thanx, Paul > > > >>> + * > >>> + * 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 > >> > > > > > -- > ~Randy