From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 11 May 2022 13:13:20 +0200 From: Petr Mladek Subject: Re: [PATCH 04/30] firmware: google: Convert regular spinlock into trylock on panic path Message-ID: <20220511111320.GB26047@pathway.suse.cz> References: <20220427224924.592546-1-gpiccoli@igalia.com> <20220427224924.592546-5-gpiccoli@igalia.com> <20220510132015.38923cb2@gandalf.local.home> <87h75xkwg9.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <87h75xkwg9.fsf@jogness.linutronix.de> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-um" Errors-To: linux-um-bounces+geert=linux-m68k.org@lists.infradead.org To: John Ogness Cc: Steven Rostedt , "Guilherme G. Piccoli" , Evan Green , Andrew Morton , bhe@redhat.com, kexec@lists.infradead.org, LKML , bcm-kernel-feedback-list@broadcom.com, linuxppc-dev@lists.ozlabs.org, linux-alpha@vger.kernel.org, linux-edac@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-leds@vger.kernel.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, Linux PM , linux-remoteproc@vger.kernel.org, linux-s390@vger.kernel.org, linux-tegra@vger.kernel.org, linux-um@lists.infradead.org, linux-xtensa@linux-xtensa.org, netdev@vger.kernel.org, openipmi-developer@lists.sourceforge.net, rcu@vger.kernel.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org, kernel-dev@igalia.com, kernel@gpiccoli.net, halves@canonical.com, fabiomirmar@gmail.com, alejandro.j.jimenez@oracle.com, Andy Shevchenko , Arnd Bergmann , Borislav Petkov , Jonathan Corbet , d.hatayama@jp.fujitsu.com, dave.hansen@linux.intel.com, dyoung@redhat.com, feng.tang@intel.com, Greg Kroah-Hartman , mikelley@microsoft.com, hidehiro.kawai.ez@hitachi.com, jgross@suse.com, Kees Cook , luto@kernel.org, mhiramat@kernel.org, mingo@redhat.com, paulmck@kernel.org, peterz@infradead.org, senozhatsky@chromium.org, Alan Stern , Thomas Gleixner , vgoyal@redhat.com, vkuznets@redhat.com, Will Deacon , Ard Biesheuvel , David Gow , Julius Werner On Tue 2022-05-10 21:46:38, John Ogness wrote: > On 2022-05-10, Steven Rostedt wrote: > >> As already mentioned in the other reply, panic() sometimes stops the > >> other CPUs using NMI, for example, see kdump_nmi_shootdown_cpus(). > >> > >> Another situation is when the CPU using the lock ends in some > >> infinite loop because something went wrong. The system is in > >> an unpredictable state during panic(). > >> > >> I am not sure if this is possible with the code under gsmi_dev.lock > >> but such things really happen during panic() in other subsystems. > >> Using trylock in the panic() code path is a good practice. > > > > I believe that Peter Zijlstra had a special spin lock for NMIs or > > early printk, where it would not block if the lock was held on the > > same CPU. That is, if an NMI happened and paniced while this lock was > > held on the same CPU, it would not deadlock. But it would block if the > > lock was held on another CPU. > > Yes. And starting with 5.19 it will be carrying the name that _you_ came > up with (cpu_sync): > > printk_cpu_sync_get_irqsave() > printk_cpu_sync_put_irqrestore() There is a risk that this lock might become a big kernel lock. This special lock would need to be used even during normal system operation. It does not make sense to suddenly start using another lock during panic. So I think that we should think twice before using it. I would prefer using trylock of the original lock when possible during panic. It is possible that I miss something. Best Regards, Petr _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um