Re: [PATCH 3.4 098/107] kernel/watchdog.c: touch_nmi_watchdog should only touch local cpu not every one

Don Zickus
2016-03-16T14:09:53Z

On Wed, Mar 16, 2016 at 04:06:32PM +0800, lizf@kernel.org wrote:
> From: Ben Zhang <benzh@chromium.org>
>
> 3.4.111-rc1 review patch. If anyone has any objections, please let me know.

Just an FYI below, this patch won't work the way it was integrated..

comments below

>
> ------------------
>
>
> commit 62572e29bc530b38921ef6059088b4788a9832a5 upstream.
>
> I ran into a scenario where while one cpu was stuck and should have
> panic'd because of the NMI watchdog, it didn't. The reason was another
> cpu was spewing stack dumps on to the console. Upon investigation, I
> noticed that when writing to the console and also when dumping the
> stack, the watchdog is touched.
>
> This causes all the cpus to reset their NMI watchdog flags and the
> 'stuck' cpu just spins forever.
>
> This change causes the semantics of touch_nmi_watchdog to be changed
> slightly. Previously, I accidentally changed the semantics and we
> noticed there was a codepath in which touch_nmi_watchdog could be
> touched from a preemtible area. That caused a BUG() to happen when
> CONFIG_DEBUG_PREEMPT was enabled. I believe it was the acpi code.
>
> My attempt here re-introduces the change to have the
> touch_nmi_watchdog() code only touch the local cpu instead of all of the
> cpus. But instead of using __get_cpu_var(), I use the
> __raw_get_cpu_var() version.
>
> This avoids the preemption problem. However my reasoning wasn't because
> I was trying to be lazy. Instead I rationalized it as, well if
> preemption is enabled then interrupts should be enabled to and the NMI
> watchdog will have no reason to trigger. So it won't matter if the
> wrong cpu is touched because the percpu interrupt counters the NMI
> watchdog uses should still be incrementing.
>
> Don said:
>
> : I'm ok with this patch, though it does alter the behaviour of how
> : touch_nmi_watchdog works. For the most part I don't think most callers
> : need to touch all of the watchdogs (on each cpu). Perhaps a corner case
> : will pop up (the scheduler?? to mimic touch_all_softlockup_watchdogs() ).
> :
> : But this does address an issue where if a system is locked up and one cpu
> : is spewing out useful debug messages (or error messages), the hard lockup
> : will fail to go off. We have seen this on RHEL also.
>
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> Signed-off-by: Ben Zhang <benzh@chromium.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> [lizf: Backported to 3.4: adjust context]
> Signed-off-by: Zefan Li <lizefan@huawei.com>
> ---
> kernel/watchdog.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 991aa93..7527c8c 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -162,6 +162,14 @@ void touch_nmi_watchdog(void)
> per_cpu(watchdog_nmi_touch, cpu) = true;
> }
> }

The above for-loop was to be replaced by the non-for-loop below.

The above for-loop is the problem this patch was solving, so keeping it
around does not solve anything. :-)

> + /*
> + * Using __raw here because some code paths have
> + * preemption enabled. If preemption is enabled
> + * then interrupts should be enabled too, in which
> + * case we shouldn't have to worry about the watchdog
> + * going off.
> + */
> + __raw_get_cpu_var(watchdog_nmi_touch) = true;
> touch_softlockup_watchdog();
> }
> EXPORT_SYMBOL(touch_nmi_watchdog);

Cheers,
Don