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McKenney" To: Steven Rostedt Cc: Joel Fernandes , Joel Fernandes , LKML , "Cc: Android Kernel" , Boqun Feng , Byungchul Park , Ingo Molnar , Masami Hiramatsu , Mathieu Desnoyers , Namhyung Kim , Peter Zijlstra , Thomas Glexiner , Tom Zanussi Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage Reply-To: paulmck@linux.vnet.ibm.com References: <20180807215522.04114097@vmware.local.home> <20180807222856.3ede96e7@vmware.local.home> <20180808130041.GI24813@linux.vnet.ibm.com> <20180808102700.38c4169d@gandalf.local.home> <20180808144200.GM24813@linux.vnet.ibm.com> <20180808112705.22e6a96d@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180808112705.22e6a96d@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18080816-0060-0000-0000-0000029A8A83 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009507; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01070999; UDB=6.00551382; IPR=6.00850540; MB=3.00022590; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-08 16:03:54 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18080816-0061-0000-0000-00004619A84F Message-Id: <20180808160350.GR24813@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-08_06:,, 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=8.0.1-1807170000 definitions=main-1808080163 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 08, 2018 at 11:27:05AM -0400, Steven Rostedt wrote: > On Wed, 8 Aug 2018 07:42:00 -0700 > "Paul E. McKenney" wrote: > > > > There's also a local_inc() if you are using per cpu pointers, that is > > > suppose to guarantee atomicity for single cpu operations. That's what > > > the ftrace ring buffer uses. > > > > Good point, that becomes atomic_long_inc() or equivalent on most > > architectures, but an incl instruction (not locked) on x86. So updating > > my earlier still-untested thought: > > > > int __srcu_read_lock_nmi(struct srcu_struct *sp) /* UNTESTED. */ > > { > > int idx; > > > > idx = READ_ONCE(sp->srcu_idx) & 0x1; > > local_inc(&sp->sda->srcu_lock_count[idx]); > > smp_mb__after_atomic(); /* B */ /* Avoid leaking critical section. */ > > return idx; > > } > > > > void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx) > > { > > smp_mb__before_atomic(); /* C */ /* Avoid leaking critical section. */ > > local_inc(&sp->sda->srcu_unlock_count[idx]); > > } > > > > Would that work, or is there a better way to handle this? > > This would work much better than using the atomic ops, and I think it > would be doable. OK, here is hoping! > I may need to revert the srcu for trace_*_rcidle() for now, as I want > most of the other changes in this merge window, and it's getting too > late to do it with these updates. Agreed, especially since I normally freeze for the next merge window shortly after -rc5. ;-) Thanx, Paul