Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event_create()

[Petr Mladek <pmladek@suse.com>]

On Thu 2021-09-16 11:57:44, Pingfan Liu wrote:
> On Wed, Sep 15, 2021 at 03:45:06PM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 15, 2021 at 11:51:00AM +0800, Pingfan Liu wrote:
> > > hardlockup_detector_event_create() indirectly calls
> > > kmem_cache_alloc_node(), which is blockable.
> > > 
> > > So here, the really planned context is is_percpu_thread().
> > > 
> > > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Wang Qing <wangqing@vivo.com>
> > > Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> > > Cc: Santosh Sivaraj <santosh@fossix.org>
> > > Cc: Sumit Garg <sumit.garg@linaro.org>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > To: linux-kernel@vger.kernel.org
> > > ---
> > >  kernel/watchdog_hld.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> > > index 247bf0b1582c..6876e796dbf5 100644
> > > --- a/kernel/watchdog_hld.c
> > > +++ b/kernel/watchdog_hld.c
> > > @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event,
> > >  
> > >  static int hardlockup_detector_event_create(void)
> > >  {
> > > -	unsigned int cpu = smp_processor_id();
> > > +	unsigned int cpu;
> > >  	struct perf_event_attr *wd_attr;
> > >  	struct perf_event *evt;
> > >  
> > > +	/* This function plans to execute in cpu bound kthread */
> > > +	BUG_ON(!is_percpu_thread());
> > > +	cpu = raw_smp_processor_id();
> > >  	wd_attr = &wd_hw_attr;
> > >  	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
> > 
> > This patch makes no sense.
> 
> This patch aims to disable any attempt such as using get_cpu()/put_cpu() to
> shut up the check_preemption_disabled().

I have to say that the description of the problem is really cryptic.
Please, provide more context, code paths, sample code, next time.

Well, I probably got it. The code might sleep. But it should run on the
same CPU even after waking up. You try to achieve this by running
the code in a process that is bound to a single CPU.

IMHO, this is not reliable. Anyone could change the affinity
of the process in the meantime.

I see two solutions. Either avoid the sleep or making sure
that the code access per-CPU variables on the same CPU
all the time.

For example, you might use

	*per_cpu_ptr(watchdog_ev, cpu) = evt;

instead of

	this_cpu_write(watchdog_ev, evt);

Best Regards,
Petr