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McKenney" To: Thomas Gleixner Cc: Sebastian Sewior , Heiko Carstens , Peter Zijlstra , Ingo Molnar , Martin Schwidefsky , LKML , linux-s390@vger.kernel.org, Stefan Liebler Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggerede Reply-To: paulmck@linux.ibm.com References: <20190130125955.GD5299@osiris> <20190130132420.spwrq2d4oxeydk5s@linutronix.de> <20190130210733.mg6aascw2gzl3oqz@linutronix.de> <20190130233557.GA4240@linux.ibm.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: 19013101-0040-0000-0000-000004BA0805 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010507; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000277; SDB=6.01154173; UDB=6.00593882; IPR=6.00934550; MB=3.00025361; MTD=3.00000008; XFM=3.00000015; UTC=2019-01-31 01:44:47 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19013101-0041-0000-0000-000008C51DC3 Message-Id: <20190131014442.GB4240@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-01-30_19:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901310011 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 31, 2019 at 12:55:18AM +0100, Thomas Gleixner wrote: > On Wed, 30 Jan 2019, Paul E. McKenney wrote: > > On Thu, Jan 31, 2019 at 12:13:51AM +0100, Thomas Gleixner wrote: > > > I might be wrong as usual, but this would definitely explain the fail very > > > well. > > > > On recent versions of GCC, the fix would be to put this between the two > > stores that need ordering: > > > > __atomic_thread_fence(__ATOMIC_RELEASE); > > > > I must defer to Heiko on whether s390 GCC might tear the stores. My > > guess is "probably not". ;-) > > So I just checked the latest glibc code. It has: > > /* We must not enqueue the mutex before we have acquired it. > Also see comments at ENQUEUE_MUTEX. */ > __asm ("" ::: "memory"); > ENQUEUE_MUTEX_PI (mutex); > /* We need to clear op_pending after we enqueue the mutex. */ > __asm ("" ::: "memory"); > THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL); > > 8f9450a0b7a9 ("Add compiler barriers around modifications of the robust mutex list.") > > in the glibc repository, There since Dec 24 2016 ... > > So the question is whether this is sufficient. That ordering only only > matters vs. the thread itself and not for others. Ah, in that case you can use __atomic_signal_fence(__ATOMIC_RELEASE) instead of the __atomic_thread_fence(__ATOMIC_RELEASE). The __atomic_thread_fence(__ATOMIC_RELEASE) provides ordering between threads, but __atomic_signal_fence(__ATOMIC_RELEASE) only does so within a thread. Thanx, Paul