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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 26 Mar 2019 23:13:16 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x2QNDFNC13041868 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 26 Mar 2019 23:13:15 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D1344B206C; Tue, 26 Mar 2019 23:13:14 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BBCB9B205F; Tue, 26 Mar 2019 23:13:14 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 26 Mar 2019 23:13:14 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 9BB4516C613D; Tue, 26 Mar 2019 16:13:15 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Neeraj Upadhyay , "Paul E . McKenney" Subject: [PATCH tip/core/rcu 13/18] rcu: Do a single rhp->func read in rcu_head_after_call_rcu() Date: Tue, 26 Mar 2019 16:13:09 -0700 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20190326231253.GA18343@linux.ibm.com> References: <20190326231253.GA18343@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 19032623-0040-0000-0000-000004D7AEA4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010820; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000282; SDB=6.01180133; UDB=6.00606765; IPR=6.00960848; MB=3.00026169; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-26 23:13:20 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19032623-0041-0000-0000-000008E2CFF0 Message-Id: <20190326231314.19518-13-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-26_15:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=15 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-1903260156 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Neeraj Upadhyay The rcu_head_after_call_rcu() function reads the rhp->func pointer twice, which can result in a false-positive WARN_ON_ONCE() if the callback were passed to call_rcu() between the two reads. Although racing rcu_head_after_call_rcu() with call_rcu() is to be a dubious use case (the return value is not reliable in that case), intermittent and irreproducible warnings are also quite dubious. This commit therefore uses a single READ_ONCE() to pick up the value of rhp->func once, then tests that value twice, thus guaranteeing consistent processing within rcu_head_after_call_rcu()(). Neverthless, racing rcu_head_after_call_rcu() with call_rcu() is still a dubious use case. Signed-off-by: Neeraj Upadhyay [ paulmck: Add blank line after declaration per checkpatch.pl. ] Signed-off-by: Paul E. McKenney --- include/linux/rcupdate.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 6cdb1db776cf..922bb6848813 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -878,9 +878,11 @@ static inline void rcu_head_init(struct rcu_head *rhp) static inline bool rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f) { - if (READ_ONCE(rhp->func) == f) + rcu_callback_t func = READ_ONCE(rhp->func); + + if (func == f) return true; - WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L); + WARN_ON_ONCE(func != (rcu_callback_t)~0L); return false; } -- 2.17.1