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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 27 Mar 2019 15:53:53 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x2RFrqtm15728684 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 27 Mar 2019 15:53:52 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 63897B2067; Wed, 27 Mar 2019 15:53:52 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 34A98B2068; Wed, 27 Mar 2019 15:53:52 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 27 Mar 2019 15:53:52 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id ED3AF16C319C; Wed, 27 Mar 2019 08:53:51 -0700 (PDT) Date: Wed, 27 Mar 2019 08:53:51 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, byungchul.park@lge.com, kernel-team@android.com, rcu@vger.kernel.org, Ingo Molnar , Josh Triplett , Lai Jiangshan , linux-kselftest@vger.kernel.org, Mathieu Desnoyers , Peter Zijlstra , Shuah Khan , Steven Rostedt , Will Deacon Subject: Re: [PATCH v2 2/4] rcutree: Add checks for dynticks counters in rcu_is_cpu_rrupt_from_idle Reply-To: paulmck@linux.ibm.com References: <20190326192411.198070-1-joel@joelfernandes.org> <20190326192411.198070-2-joel@joelfernandes.org> <20190327024751.GV4102@linux.ibm.com> <20190327153401.GA152912@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190327153401.GA152912@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19032715-0040-0000-0000-000004D81015 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010824; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000282; SDB=6.01180464; UDB=6.00606765; IPR=6.00961182; MB=3.00026182; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-27 15:53:57 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19032715-0041-0000-0000-000008E331F0 Message-Id: <20190327155351.GZ4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-27_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-1903270111 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Wed, Mar 27, 2019 at 11:34:01AM -0400, Joel Fernandes wrote: > On Tue, Mar 26, 2019 at 07:47:51PM -0700, Paul E. McKenney wrote: > > On Tue, Mar 26, 2019 at 03:24:09PM -0400, Joel Fernandes (Google) wrote: > > > In the future we would like to combine the dynticks and dynticks_nesting > > > counters thus leading to simplifying the code. At the moment we cannot > > > do that due to concerns about usermode upcalls appearing to RCU as half > > > of an interrupt. Byungchul tried to do it in [1] but the > > > "half-interrupt" concern was raised. It is half because, what RCU > > > expects is rcu_irq_enter() and rcu_irq_exit() pairs when the usermode > > > exception happens. However, only rcu_irq_enter() is observed. This > > > concern may not be valid anymore, but at least it used to be the case. > > > > > > Out of abundance of caution, Paul added warnings [2] in the RCU code > > > which if not fired by 2021 may allow us to assume that such > > > half-interrupt scenario cannot happen any more, which can lead to > > > simplification of this code. > > > > > > Summary of the changes are the following: > > > > > > (1) In preparation for this combination of counters in the future, we > > > first need to first be sure that rcu_rrupt_from_idle cannot be called > > > from anywhere but a hard-interrupt because previously, the comments > > > suggested otherwise so let us be sure. We discussed this here [3]. We > > > use the services of lockdep to accomplish this. > > > > > > (2) Further rcu_rrupt_from_idle() is not explicit about how it is using > > > the counters which can lead to weird future bugs. This patch therefore > > > makes it more explicit about the specific counter values being tested > > > > > > (3) Lastly, we check for counter underflows just to be sure these are > > > not happening, because the previous code in rcu_rrupt_from_idle() was > > > allowing the case where the counters can underflow, and the function > > > would still return true. Now we are checking for specific values so let > > > us be confident by additional checking, that such underflows don't > > > happen. Any case, if they do, we should fix them and the screaming > > > warning is appropriate. All these checks checks are NOOPs if PROVE_RCU > > > and PROVE_LOCKING are disabled. > > > > > > [1] https://lore.kernel.org/patchwork/patch/952349/ > > > [2] Commit e11ec65cc8d6 ("rcu: Add warning to detect half-interrupts") > > > [3] https://lore.kernel.org/lkml/20190312150514.GB249405@google.com/ > > > > > > Cc: byungchul.park@lge.com > > > Cc: kernel-team@android.com > > > Cc: rcu@vger.kernel.org > > > Signed-off-by: Joel Fernandes (Google) > > > > Color me stupid: > > > > [ 48.845724] ------------[ cut here ]------------ > > [ 48.846619] Not in hardirq as expected > > [ 48.847322] WARNING: CPU: 5 PID: 34 at /home/git/linux-2.6-tip/kernel/rcu/tree.c:388 rcu_is_cpu_rrupt_from_idle+0xea/0x110 > > [ 48.849302] Modules linked in: > > [ 48.849869] CPU: 5 PID: 34 Comm: cpuhp/5 Not tainted 5.1.0-rc1+ #1 > > [ 48.850985] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > > [ 48.852436] RIP: 0010:rcu_is_cpu_rrupt_from_idle+0xea/0x110 > > [ 48.853455] Code: 85 c0 0f 85 59 ff ff ff 80 3d 33 55 68 01 00 0f 85 4c ff ff ff 48 c7 c7 48 d8 cc 8e 31 c0 c6 05 1d 55 68 01 01 e8 66 54 f8 ff <0f> 0b e9 30 ff ff ff 65 48 8b 05 df 58 54 72 48 85 c0 0f 94 c0 0f > > [ 48.856783] RSP: 0000:ffffbc46802dfdc0 EFLAGS: 00010082 > > [ 48.857735] RAX: 000000000000001a RBX: 0000000000022b80 RCX: 0000000000000000 > > [ 48.859028] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8dac906c > > [ 48.860313] RBP: ffffbc46802dfe20 R08: 0000000000000001 R09: 0000000000000001 > > [ 48.861607] R10: 000000007d53d16d R11: ffffbc46802dfb48 R12: ffff9e7d7eb62b80 > > [ 48.862898] R13: 0000000000000005 R14: ffffffff8dae2ac0 R15: 00000000000000c9 > > [ 48.864191] FS: 0000000000000000(0000) GS:ffff9e7d7eb40000(0000) knlGS:0000000000000000 > > [ 48.865663] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > [ 48.866702] CR2: 0000000000000000 CR3: 0000000021022000 CR4: 00000000000006e0 > > [ 48.867993] Call Trace: > > [ 48.868450] rcu_exp_handler+0x35/0x90 > > [ 48.869147] generic_exec_single+0xab/0x100 > > [ 48.869918] ? rcu_barrier+0x240/0x240 > > [ 48.870607] smp_call_function_single+0x8e/0xd0 > > [ 48.871441] rcutree_online_cpu+0x80/0x90 > > [ 48.872181] cpuhp_invoke_callback+0xb5/0x890 > > [ 48.872979] cpuhp_thread_fun+0x172/0x210 > > [ 48.873722] ? cpuhp_thread_fun+0x2a/0x210 > > [ 48.874474] smpboot_thread_fn+0x10d/0x160 > > [ 48.875224] kthread+0xf3/0x130 > > [ 48.875804] ? sort_range+0x20/0x20 > > [ 48.876446] ? kthread_cancel_delayed_work_sync+0x10/0x10 > > [ 48.877445] ret_from_fork+0x3a/0x50 > > [ 48.878124] irq event stamp: 734 > > [ 48.878717] hardirqs last enabled at (733): [] _raw_spin_unlock_irqrestore+0x2d/0x40 > > [ 48.880402] hardirqs last disabled at (734): [] generic_exec_single+0x9a/0x100 > > [ 48.881986] softirqs last enabled at (0): [] copy_process.part.56+0x61f/0x2110 > > [ 48.883540] softirqs last disabled at (0): [<0000000000000000>] (null) > > [ 48.884840] ---[ end trace 00b4c1d2f816f4ed ]--- > > > > If a CPU invokes generic_exec_single() on itself, the "IPI handler" will > > be invoked directly, triggering your new lockdep check. Which is a bit > > wasteful. My thought is to add code to sync_rcu_exp_select_node_cpus() > > to check the CPU with preemption disabled, avoiding the call to > > smp_call_function_single() in that case. > > > > I have queued all four of your patches, and am trying the fix to > > the caller of smp_call_function_single() shown below. Thoughts? > > Oh interesting. Your fix makes sense. I will go through these paths more as > well since I'm not super familiar with this area of the RCU code. But I had > one small nit below. Very good, applying that change. I have a similar issue in the CPU-hotplug code that I will also be fixing. Are there other places where I should be using get_cpu()? > Also thanks for pulling the patches, I tested TREE09 and TASKS02 which > disable SMP and both passed. Sounds good! Thanx, Paul > > ------------------------------------------------------------------------ > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 9c990df880d1..51d61028abf1 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -384,7 +384,13 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > > mask_ofl_test |= mask; > > continue; > > } > > + preempt_disable(); > > + if (smp_processor_id() == cpu) { > > Can be this? > if (get_cpu() == cpu) { > put_cpu(); > continue; > } > > > + preempt_enable(); > > + continue; > > + } > > ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0); > > + preempt_enable(); > > and here: > put_cpu(); > > > if (!ret) { > > mask_ofl_ipi &= ~mask; > > continue; > > > > thanks, > > - Joel >