Re: [PATCH v2] locking/lock_events: Use this_cpu_add() when necessary

[Will Deacon]

On Fri, May 24, 2019 at 12:53:46PM -0400, Waiman Long wrote:
> The kernel test robot has reported that the use of __this_cpu_add()
> causes bug messages like:
> 
>   BUG: using __this_cpu_add() in preemptible [00000000] code: ...
> 
> This is only an issue on preempt kernel where preemption can happen in
> the middle of a percpu operation. We are still using __this_cpu_*() for
> !preempt kernel to avoid additional overhead in case CONFIG_PREEMPT_COUNT
> is set.
> 
>  v2: Simplify the condition to just preempt or !preempt.
> 
> Fixes: a8654596f0371 ("locking/rwsem: Enable lock event counting")
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/locking/lock_events.h | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h
> index feb1acc54611..05f34068ec06 100644
> --- a/kernel/locking/lock_events.h
> +++ b/kernel/locking/lock_events.h
> @@ -30,13 +30,32 @@ enum lock_events {
>   */
>  DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
>  
> +/*
> + * The purpose of the lock event counting subsystem is to provide a low
> + * overhead way to record the number of specific locking events by using
> + * percpu counters. It is the percpu sum that matters, not specifically
> + * how many of them happens in each cpu.
> + *
> + * In !preempt kernel, we can just use __this_cpu_*() as preemption
> + * won't happen in the middle of the percpu operation. In preempt kernel,
> + * preemption happens in the middle of the percpu operation may produce
> + * incorrect result.
> + */
> +#ifdef CONFIG_PREEMPT
> +#define lockevent_percpu_inc(x)		this_cpu_inc(x)
> +#define lockevent_percpu_add(x, v)	this_cpu_add(x, v)
> +#else
> +#define lockevent_percpu_inc(x)		__this_cpu_inc(x)
> +#define lockevent_percpu_add(x, v)	__this_cpu_add(x, v)

Are you sure this works wrt IRQs? For example, if I take an interrupt when
trying to update the counter, and then the irq handler takes a qspinlock
which in turn tries to update the counter. Would I lose an update in that
scenario?

Will