From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752234AbeDFVGs (ORCPT ); Fri, 6 Apr 2018 17:06:48 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:54886 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752100AbeDFVGr (ORCPT ); Fri, 6 Apr 2018 17:06:47 -0400 Date: Fri, 6 Apr 2018 14:07:41 -0700 From: "Paul E. McKenney" To: Randy Dunlap Cc: Andrea Parri , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18040621-0036-0000-0000-000002DD0CB6 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008813; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000256; SDB=6.01014106; UDB=6.00516972; IPR=6.00793366; MB=3.00020452; MTD=3.00000008; XFM=3.00000015; UTC=2018-04-06 21:06:45 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18040621-0037-0000-0000-000043E45035 Message-Id: <20180406210741.GF3948@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-04-06_11:,, 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-1804060212 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? 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 >