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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Petr, On Tue, Apr 26, 2022 at 10:19:02AM +0200, Petr Mladek wrote: > On Sun 2022-04-24 15:19:52, Huang Shijie wrote: > > If kdump is configured, nmi_panic() may run to machine_kexec(). > > > > In NMI context, the defer_console_output() defers the console > > output by using wake_up_klogd to flush the printk ringbuffer > > to console. > > > > But in the machine_kexec, the system will reset, and there is > > no chance for the wake_up_klogd to do its job. So we can _not_ > > see any log on the console since the nmi_panic > > (nmi_panic() will disable the irq). > > > > This patch fixes this issue by using console_flush_on_panic() > > to flush to console. > > > > After this patch, we can see all the log since the nmi_panic > > in the panic console. > > This is not a good idea. The crashdump is the best source of > information about the crashed system. It includes the complete > log. okay. Sometimes, we cannot get the crashdump file, so any log is important to us. > > The system is in unknown state during panic(). Any operation > might break. Flushing consoles increases the risk that > the crashdump will not get generated. The crashdump is more > important. If the crashdump succeeds than the consoles are > not needed. > > Note that printk() does not handle consoles in NMI because it might > cause deadlock. console_flush_on_panic() tries to avoid deadlock > caused by console_sem. Also the particular console drivers are > more careful because oops_in_progress is set at this stage. > But there is still a risk of the deadlock. There might be another > locks that are do not check oops_in_progress. Also a potential > double unlock might cause deadlock. okay, thanks for the detail explanations. > > IMHO, the main motivation for this patch was to flush the per-CPU > printk buffers (v1). But it is not longer needed. The buffers > were removed in 5.15-rc1, see the commit 93d102f094be9beab28e > ("printk: remove safe buffers"). > > The only reason to call console drivers when crashdump is generated > is to debug the kexec code path. But I am not sure if > console_flush_on_panic() would help here. The kexec might fail > anytime before or after this flush so that the important > messages will not be visible anyway. John Ogness is going > to add atomic serial console that might be better for this > use case. I hope it is ready as soon as possible.. Thanks Huang Shijie _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel