Messages in this thread | | | From | "Hoeun Ryu" <> | Subject | RE: [PATCH] printk: make printk_safe_flush safe in NMI context by skipping flushing | Date | Wed, 30 May 2018 07:43:32 +0900 |
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> -----Original Message----- > From: Sergey Senozhatsky [mailto:sergey.senozhatsky.work@gmail.com] > Sent: Tuesday, May 29, 2018 9:13 PM > To: Hoeun Ryu <hoeun.ryu@lge.com.com> > Cc: Petr Mladek <pmladek@suse.com>; Sergey Senozhatsky > <sergey.senozhatsky@gmail.com>; Steven Rostedt <rostedt@goodmis.org>; > Hoeun Ryu <hoeun.ryu@lge.com>; linux-kernel@vger.kernel.org > Subject: Re: [PATCH] printk: make printk_safe_flush safe in NMI context by > skipping flushing > > On (05/29/18 11:51), Hoeun Ryu wrote: > > Make printk_safe_flush() safe in NMI context. > > nmi_trigger_cpumask_backtrace() can be called in NMI context. For > example the > > function is called in watchdog_overflow_callback() if the flag of > hardlockup > > backtrace (sysctl_hardlockup_all_cpu_backtrace) is true and > > watchdog_overflow_callback() function is called in NMI context on some > > architectures. > > Calling printk_safe_flush() in nmi_trigger_cpumask_backtrace() > eventually tries > > to lock logbuf_lock in vprintk_emit() but the logbuf_lock can be already > locked in > > preempted contexts (task or irq in this case) or by other CPUs and it > may cause > > deadlocks. > > By making printk_safe_flush() safe in NMI context, the backtrace > triggering CPU > > just skips flushing if the lock is not avaiable in NMI context. The > messages in > > per-cpu nmi buffer of the backtrace triggering CPU can be lost if the > CPU is in > > hard lockup (because irq is disabled here) but if panic() is not called. > The > > flushing can be delayed by the next irq work in normal cases. > > Any chance we can add more info to the commit message? E.g. backtraces > which would describe "how" is this possible (like the one I posted in > another email). Just to make it more clear. >
OK, I will.
> > @@ -254,6 +254,16 @@ void printk_safe_flush(void) > > { > > int cpu; > > > > + /* > > + * Just avoid deadlocks here, we could loose the messages in per- > cpu nmi buffer > > + * in the case that hardlockup happens but panic() is not called > (irq_work won't > > + * work). > > + * The flushing can be delayed by the next irq_work if flushing is > skippped here > ^^ skipped >
Typo will be fixed.
> -ss
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