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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 6 Jun 2019 14:51:50 +0100 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 x56Dpn7V34013514 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 6 Jun 2019 13:51:49 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D9604B2064; Thu, 6 Jun 2019 13:51:49 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A79A0B205F; Thu, 6 Jun 2019 13:51:49 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.209.205]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 6 Jun 2019 13:51:49 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 279D516C5DA1; Thu, 6 Jun 2019 06:51:49 -0700 (PDT) Date: Thu, 6 Jun 2019 06:51:49 -0700 From: "Paul E. McKenney" To: Herbert Xu Cc: rcu@vger.kernel.org, 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, torvalds@linux-foundation.org Subject: Re: [PATCH RFC tip/core/rcu] Restore barrier() to rcu_read_lock() and rcu_read_unlock() Reply-To: paulmck@linux.ibm.com References: <20190606131933.GA12576@linux.ibm.com> <20190606134233.saqa3insjv75xu6o@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190606134233.saqa3insjv75xu6o@gondor.apana.org.au> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19060613-0052-0000-0000-000003CC3CE1 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011223; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01214074; UDB=6.00638159; IPR=6.00995154; MB=3.00027206; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-06 13:51:55 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19060613-0053-0000-0000-00006135288C Message-Id: <20190606135149.GM28207@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-06_10:,, 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-1906060099 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 06, 2019 at 09:42:33PM +0800, Herbert Xu wrote: > On Thu, Jun 06, 2019 at 06:19:33AM -0700, Paul E. McKenney wrote: > > Commit bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree > > RCU readers") removed the barrier() calls from rcu_read_lock() and > > rcu_write_lock() in CONFIG_PREEMPT=n&&CONFIG_PREEMPT_COUNT=n kernels. > > Within RCU, this commit was OK, but it failed to account for things like > > get_user() that can pagefault and that can be reordered by the compiler. > > Lack of the barrier() calls in rcu_read_lock() and rcu_read_unlock() > > can cause these page faults to migrate into RCU read-side critical > > sections, which in CONFIG_PREEMPT=n kernels could result in too-short > > grace periods and arbitrary misbehavior. Please see commit 386afc91144b > > ("spinlocks and preemption points need to be at least compiler barriers") > > for more details. > > > > This commit therefore restores the barrier() call to both rcu_read_lock() > > and rcu_read_unlock(). It also removes them from places in the RCU update > > machinery that used to need compensatory barrier() calls, effectively > > reverting commit bb73c52bad36 ("rcu: Don't disable preemption for Tiny > > and Tree RCU readers"). > > > > Reported-by: Herbert Xu > > Reported-by: Linus Torvalds > > Signed-off-by: Paul E. McKenney > > Paul, Linus has already commited his patch: > > commit 66be4e66a7f422128748e3c3ef6ee72b20a6197b > Author: Linus Torvalds > Date: Mon Jun 3 13:26:20 2019 -0700 > > rcu: locking and unlocking need to always be at least barriers > > So you'll need to rebase this. Thank you for letting me know. Easy enough to do, just remove those two hunks from my patch. Thanx, Paul