Messages in this thread Patch in this message | | | Date | Fri, 16 Dec 2011 16:52:07 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] panic: Don't print redundant backtraces on oops |
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On Wed, 7 Dec 2011 16:36:43 -0800 Andi Kleen <andi@firstfloor.org> wrote:
> From: Andi Kleen <ak@linux.intel.com> > > When an oops causes a panic and panic prints another backtrace it's > pretty common to have the original oops data be scrolled away on a 80x50 > screen. > > The second backtrace is quite redundant and not needed anyways. > > So don't print the panic backtrace when oops_in_progress is true. > > Signed-off-by: Andi Kleen <ak@linux.intel.com> > --- > kernel/panic.c | 3 ++- > 1 files changed, 2 insertions(+), 1 deletions(-) > > diff --git a/kernel/panic.c b/kernel/panic.c > index b2659360..398412b 100644 > --- a/kernel/panic.c > +++ b/kernel/panic.c > @@ -78,7 +78,8 @@ NORET_TYPE void panic(const char * fmt, ...) > va_end(args); > printk(KERN_EMERG "Kernel panic - not syncing: %s\n",buf); > #ifdef CONFIG_DEBUG_BUGVERBOSE > - dump_stack(); > + if (!oops_in_progress) > + dump_stack(); > #endif
This is kinda related to Michael's kdump-fix-crash_kexec-smp_send_stop-race-in-panic.patch, below.
afacit Michael's patch will prevent panic-within-panic, and it does this by accident becasue we never thought about it. But it won't fix panic-within-other-oops.
Is there some clever way in which we can satisfy both requirements in one hit?
From: Michael Holzheu <holzheu@linux.vnet.ibm.com> Subject: kdump: fix crash_kexec()/smp_send_stop() race in panic()
When two CPUs call panic at the same time there is a possible race condition that can stop kdump. The first CPU calls crash_kexec() and the second CPU calls smp_send_stop() in panic() before crash_kexec() finished on the first CPU. So the second CPU stops the first CPU and therefore kdump fails:
1st CPU: panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump
2nd CPU: panic()->crash_kexec()->kexec_mutex already held by 1st CPU ->smp_send_stop()-> stop 1st CPU (stop kdump)
This patch fixes the problem by introducing a spinlock in panic that allows only one CPU to process crash_kexec() and the subsequent panic code.
All other CPUs call the weak function panic_smp_self_stop() that stops the CPU itself. This function can be overloaded by architecture code. For example "tile" can use their lower-power "nap" instruction for that.
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com> Acked-by: Chris Metcalf <cmetcalf@tilera.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> ---
kernel/panic.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)
diff -puN kernel/panic.c~kdump-fix-crash_kexec-smp_send_stop-race-in-panic kernel/panic.c --- a/kernel/panic.c~kdump-fix-crash_kexec-smp_send_stop-race-in-panic +++ a/kernel/panic.c @@ -49,6 +49,15 @@ static long no_blink(int state) long (*panic_blink)(int state); EXPORT_SYMBOL(panic_blink); +/* + * Stop ourself in panic -- architecture code may override this + */ +void __weak panic_smp_self_stop(void) +{ + while (1) + cpu_relax(); +} + /** * panic - halt the system * @fmt: The text string to print @@ -59,6 +68,7 @@ EXPORT_SYMBOL(panic_blink); */ void panic(const char *fmt, ...) { + static DEFINE_SPINLOCK(panic_lock); static char buf[1024]; va_list args; long i, i_next = 0; @@ -68,8 +78,14 @@ void panic(const char *fmt, ...) * It's possible to come here directly from a panic-assertion and * not have preempt disabled. Some functions called from here want * preempt to be disabled. No point enabling it later though... + * + * Only one CPU is allowed to execute the panic code from here. For + * multiple parallel invocations of panic, all other CPUs either + * stop themself or will wait until they are stopped by the 1st CPU + * with smp_send_stop(). */ - preempt_disable(); + if (!spin_trylock(&panic_lock)) + panic_smp_self_stop(); console_verbose(); bust_spinlocks(1); _
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