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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 12 Nov 2018 02:20:09 -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 wAC2K8f829819024 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 12 Nov 2018 02:20:08 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4446FB2064; Mon, 12 Nov 2018 02:20:08 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0D19BB205F; Mon, 12 Nov 2018 02:20:08 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.207.24]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 12 Nov 2018 02:20:07 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 7BA4616C5D4F; Sun, 11 Nov 2018 18:20:09 -0800 (PST) Date: Sun, 11 Nov 2018 18:20:09 -0800 From: "Paul E. McKenney" To: Steven Rostedt 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, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 1/4] rcu: Eliminate BUG_ON() for sync.c Reply-To: paulmck@linux.ibm.com References: <20181111193156.GA3666@linux.ibm.com> <20181111193217.4010-1-paulmck@linux.ibm.com> <20181111210704.7bb9946f@vmware.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181111210704.7bb9946f@vmware.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18111202-2213-0000-0000-00000315E6EF X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010030; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116143; UDB=6.00578812; IPR=6.00896251; MB=3.00024116; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-12 02:20:13 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111202-2214-0000-0000-00005C393480 Message-Id: <20181112022009.GC4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-12_01:,, 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-1811120019 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 11, 2018 at 09:07:04PM -0500, Steven Rostedt wrote: > On Sun, 11 Nov 2018 11:32:14 -0800 > "Paul E. McKenney" wrote: > > > The sync.c file has a number of calls to BUG_ON(), which panics the > > kernel, which is not a good strategy for devices (like embedded) that > > don't have a way to capture console output. This commit therefore > > changes these BUG_ON() calls to WARN_ON_ONCE(), but does so quite naively. > > > > Reported-by: Linus Torvalds > > Signed-off-by: Paul E. McKenney > > Acked-by: Oleg Nesterov > > Cc: Peter Zijlstra > > --- > > kernel/rcu/sync.c | 13 ++++++------- > > 1 file changed, 6 insertions(+), 7 deletions(-) > > > > diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c > > index 3f943efcf61c..a6ba446a9693 100644 > > --- a/kernel/rcu/sync.c > > +++ b/kernel/rcu/sync.c > > @@ -125,8 +125,7 @@ void rcu_sync_enter(struct rcu_sync *rsp) > > rsp->gp_state = GP_PENDING; > > spin_unlock_irq(&rsp->rss_lock); > > > > - BUG_ON(need_wait && need_sync); > > - > > + WARN_ON_ONCE(need_wait && need_sync); > > if (need_sync) { > > gp_ops[rsp->gp_type].sync(); > > rsp->gp_state = GP_PASSED; > > @@ -139,7 +138,7 @@ void rcu_sync_enter(struct rcu_sync *rsp) > > * Nobody has yet been allowed the 'fast' path and thus we can > > * avoid doing any sync(). The callback will get 'dropped'. > > */ > > - BUG_ON(rsp->gp_state != GP_PASSED); > > + WARN_ON_ONCE(rsp->gp_state != GP_PASSED); > > } > > } > > > > @@ -166,8 +165,8 @@ static void rcu_sync_func(struct rcu_head *rhp) > > struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head); > > unsigned long flags; > > > > - BUG_ON(rsp->gp_state != GP_PASSED); > > - BUG_ON(rsp->cb_state == CB_IDLE); > > + WARN_ON_ONCE(rsp->gp_state != GP_PASSED); > > + WARN_ON_ONCE(rsp->cb_state == CB_IDLE); > > > > spin_lock_irqsave(&rsp->rss_lock, flags); > > if (rsp->gp_count) { > > @@ -225,7 +224,7 @@ void rcu_sync_dtor(struct rcu_sync *rsp) > > { > > int cb_state; > > > > - BUG_ON(rsp->gp_count); > > + WARN_ON_ONCE(rsp->gp_count); > > > > spin_lock_irq(&rsp->rss_lock); > > if (rsp->cb_state == CB_REPLAY) > > @@ -235,6 +234,6 @@ void rcu_sync_dtor(struct rcu_sync *rsp) > > > > if (cb_state != CB_IDLE) { > > gp_ops[rsp->gp_type].wait(); > > - BUG_ON(rsp->cb_state != CB_IDLE); > > + WARN_ON_ONCE(rsp->cb_state != CB_IDLE); > > } > > } > > I take it that if any of these WARN_ON_ONCE() triggers, they wont cause > immediate catastrophe, and/or there's no gentle way out like you have > with the other patches exiting the function when one is hit. Oleg was actually OK with removing them entirely: "I added these BUG_ON's for documentation when I was prototyping this code, perhaps we can simply remove them." And they are "cannot happen" types of things (famous last words). Oleg also has another approach that could rip-and-replace the current implementation, which would render these WARN*()s moot. Thanx, Paul