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Tue, 26 Oct 2021 20:29:15 -0700 (PDT) Received: from localhost ([118.208.159.180]) by smtp.gmail.com with ESMTPSA id m2sm20384044pgd.70.2021.10.26.20.29.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 26 Oct 2021 20:29:15 -0700 (PDT) Date: Wed, 27 Oct 2021 13:29:10 +1000 From: Nicholas Piggin Subject: Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock To: benh@kernel.crashing.org, Laurent Dufour , mpe@ellerman.id.au, paulus@samba.org References: <20211026162740.16283-1-ldufour@linux.ibm.com> <20211026162740.16283-2-ldufour@linux.ibm.com> In-Reply-To: <20211026162740.16283-2-ldufour@linux.ibm.com> MIME-Version: 1.0 Message-Id: <1635303699.wgz87uxy4c.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am: > When handling the Watchdog interrupt, long processing should not be done > while holding the __wd_smp_lock. This prevents the other CPUs to grab it > and to process Watchdog timer interrupts. Furhtermore, this could lead to > the following situation: >=20 > CPU x detect lockup on CPU y and grab the __wd_smp_lock > in watchdog_smp_panic() > CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock > in soft_nmi_interrupt() > CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi() CPU y should get the IPI here if it's a NMI IPI (which will be true for=20 >=3D POWER9 64s). That said, not all platforms support it and the console lock problem seems real, so okay. > CPU x will timeout and so has spent 1s waiting while holding the > __wd_smp_lock. >=20 > A deadlock may also happen between the __wd_smp_lock and the console_owne= r > 'lock' this way: > CPU x grab the console_owner > CPU y grab the __wd_smp_lock > CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock > CPU y wants to print something and wait for console_owner > -> deadlock >=20 > Doing all the long processing without holding the _wd_smp_lock prevents > these situations. The intention was to avoid logs getting garbled e.g., if multiple=20 different CPUs fire at once. I wonder if instead we could deal with that by protecting the IPI=20 sending and printing stuff with a trylock, and if you don't get the trylock then just return, and you'll come back with the next timer interrupt. Thanks, Nick >=20 > Signed-off-by: Laurent Dufour > --- > arch/powerpc/kernel/watchdog.c | 31 +++++++++++++++++-------------- > 1 file changed, 17 insertions(+), 14 deletions(-) >=20 > diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdo= g.c > index f9ea0e5357f9..bc7411327066 100644 > --- a/arch/powerpc/kernel/watchdog.c > +++ b/arch/powerpc/kernel/watchdog.c > @@ -149,6 +149,8 @@ static void set_cpu_stuck(int cpu, u64 tb) > =20 > static void watchdog_smp_panic(int cpu, u64 tb) > { > + cpumask_t cpus_pending_copy; > + u64 last_reset_tb_copy; > unsigned long flags; > int c; > =20 > @@ -161,29 +163,32 @@ static void watchdog_smp_panic(int cpu, u64 tb) > if (cpumask_weight(&wd_smp_cpus_pending) =3D=3D 0) > goto out; > =20 > + cpumask_copy(&cpus_pending_copy, &wd_smp_cpus_pending); > + last_reset_tb_copy =3D wd_smp_last_reset_tb; > + > + /* Take the stuck CPUs out of the watch group */ > + set_cpumask_stuck(&wd_smp_cpus_pending, tb); > + > + wd_smp_unlock(&flags); > + > pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n", > - cpu, cpumask_pr_args(&wd_smp_cpus_pending)); > + cpu, cpumask_pr_args(&cpus_pending_copy)); > pr_emerg("CPU %d TB:%lld, last SMP heartbeat TB:%lld (%lldms ago)\n", > - cpu, tb, wd_smp_last_reset_tb, > - tb_to_ns(tb - wd_smp_last_reset_tb) / 1000000); > + cpu, tb, last_reset_tb_copy, > + tb_to_ns(tb - last_reset_tb_copy) / 1000000); > =20 > if (!sysctl_hardlockup_all_cpu_backtrace) { > /* > * Try to trigger the stuck CPUs, unless we are going to > * get a backtrace on all of them anyway. > */ > - for_each_cpu(c, &wd_smp_cpus_pending) { > + for_each_cpu(c, &cpus_pending_copy) { > if (c =3D=3D cpu) > continue; > smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000); > } > } > =20 > - /* Take the stuck CPUs out of the watch group */ > - set_cpumask_stuck(&wd_smp_cpus_pending, tb); > - > - wd_smp_unlock(&flags); > - > if (sysctl_hardlockup_all_cpu_backtrace) > trigger_allbutself_cpu_backtrace(); > =20 > @@ -204,6 +209,8 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb) > unsigned long flags; > =20 > wd_smp_lock(&flags); > + cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck); > + wd_smp_unlock(&flags); > =20 > pr_emerg("CPU %d became unstuck TB:%lld\n", > cpu, tb); > @@ -212,9 +219,6 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb) > show_regs(regs); > else > dump_stack(); > - > - cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck); > - wd_smp_unlock(&flags); > } > return; > } > @@ -267,6 +271,7 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt) > return 0; > } > set_cpu_stuck(cpu, tb); > + wd_smp_unlock(&flags); > =20 > pr_emerg("CPU %d self-detected hard LOCKUP @ %pS\n", > cpu, (void *)regs->nip); > @@ -277,8 +282,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt) > print_irqtrace_events(current); > show_regs(regs); > =20 > - wd_smp_unlock(&flags); > - > if (sysctl_hardlockup_all_cpu_backtrace) > trigger_allbutself_cpu_backtrace(); > =20 > --=20 > 2.33.1 >=20 >=20