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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 28 Jan 2019 16:23:19 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x0SGNIja19267692 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 28 Jan 2019 16:23:18 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7698EB2064; Mon, 28 Jan 2019 16:23:18 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5917BB205F; Mon, 28 Jan 2019 16:23:18 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.57]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 28 Jan 2019 16:23:18 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 29BD516C5F77; Mon, 28 Jan 2019 08:23:18 -0800 (PST) Date: Mon, 28 Jan 2019 08:23:18 -0800 From: "Paul E. McKenney" To: Sasha Levin Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH AUTOSEL 4.20 028/304] srcu: Prevent __call_srcu() counter wrap with read-side critical section Reply-To: paulmck@linux.ibm.com References: <20190128154341.47195-1-sashal@kernel.org> <20190128154341.47195-28-sashal@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190128154341.47195-28-sashal@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19012816-0068-0000-0000-0000038A1CF1 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010493; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000277; SDB=6.01153048; UDB=6.00601113; IPR=6.00933410; MB=3.00025324; MTD=3.00000008; XFM=3.00000015; UTC=2019-01-28 16:23:20 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19012816-0069-0000-0000-0000474CA849 Message-Id: <20190128162318.GS4240@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-01-28_09:,, 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=1031 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-1901280123 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 28, 2019 at 10:39:05AM -0500, Sasha Levin wrote: > From: "Paul E. McKenney" > > [ Upstream commit 0607ba8403c4cdb253f8c5200ecf654dfb7790cc ] > > Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from > both process and interrupt context"), it has been permissible > to use SRCU read-side critical sections in interrupt context. > This allows __call_srcu() to use SRCU read-side critical sections to > prevent a new SRCU grace period from ending before the call to either > srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing > SRCU grace-period counter overflow during that time. > > Note that this does not permit removal of the counter-wrap checks in > srcu_gp_end(). These check are necessary to handle the case where > a given CPU does not interact at all with SRCU for an extended time > period. > > This commit therefore adds an SRCU read-side critical section to > __call_srcu() in order to prevent grace period counter wrap during > the funnel-locking process. > > Signed-off-by: Paul E. McKenney > Signed-off-by: Sasha Levin Hello, Sasha, I recommend -against- backporting this one. It is a theoretical bug that requires rather long preemption, so the risk of backporting likely greatly exceeeds the risk of the bug actually happening, especially on 64-bit systems. Thanx, Paul > --- > kernel/rcu/srcutree.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index a180abc8c925..697a2d7e8e8a 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -860,6 +860,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp, > rcu_callback_t func, bool do_norm) > { > unsigned long flags; > + int idx; > bool needexp = false; > bool needgp = false; > unsigned long s; > @@ -873,6 +874,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp, > return; > } > rhp->func = func; > + idx = srcu_read_lock(sp); > local_irq_save(flags); > sdp = this_cpu_ptr(sp->sda); > spin_lock_rcu_node(sdp); > @@ -894,6 +896,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp, > srcu_funnel_gp_start(sp, sdp, s, do_norm); > else if (needexp) > srcu_funnel_exp_start(sp, sdp->mynode, s); > + srcu_read_unlock(sp, idx); > } > > /** > -- > 2.19.1 >