From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753074AbeDHVNx (ORCPT ); Sun, 8 Apr 2018 17:13:53 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:43476 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752720AbeDHVNv (ORCPT ); Sun, 8 Apr 2018 17:13:51 -0400 Date: Sun, 8 Apr 2018 14:14:46 -0700 From: "Paul E. McKenney" To: Andrea Parri Cc: Randy Dunlap , 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() Reply-To: paulmck@linux.vnet.ibm.com 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> <20180406215825.GA3646@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180406215825.GA3646@andrea> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18040821-0008-0000-0000-000002F2F917 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008820; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000256; SDB=6.01015068; UDB=6.00517536; IPR=6.00794319; MB=3.00020476; MTD=3.00000008; XFM=3.00000015; UTC=2018-04-08 21:13:48 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18040821-0009-0000-0000-000038D3FC90 Message-Id: <20180408211446.GU3948@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-04-08_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=3 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1804080232 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 06, 2018 at 11:58:25PM +0200, Andrea Parri wrote: > 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. Are you sure? $ git grep "\* Returns:" | wc -l 2470 $ git grep "\* Return:" | wc -l 4144 Looks like more than a third of them are already "Returns:". ;-) > 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. If Randy is certain that he would like to continue propagating this grammatical infelicity, sure. ;-) Thanx, Paul > 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 >