Re: [PATCH] events: Annotate parent_ctx with __rcu

[Peter Zijlstra]

On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> parent_ctx is used under RCU context in kernel/events/core.c,
> tell sparse about it aswell.
> 
> Fixes the following instances of sparse error:
> kernel/events/core.c:3221:18: error: incompatible types in comparison
> kernel/events/core.c:3222:23: error: incompatible types in comparison
> 
> This introduces the following two sparse errors:
> kernel/events/core.c:3117:18: error: incompatible types in comparison
> kernel/events/core.c:3121:30: error: incompatible types in comparison
> 
> Hence, use rcu_dereference() to access parent_ctx and silence
> the newly introduced errors.
> 
> Signed-off-by: Amol Grover <frextrite@xxxxxxxxx>
> ---
>  include/linux/perf_event.h |  2 +-
>  kernel/events/core.c       | 11 ++++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 6d4c22aee384..e18a7bdc6f98 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -810,7 +810,7 @@ struct perf_event_context {
>  	 * These fields let us detect when two contexts have both
>  	 * been cloned (inherited) from a common ancestor.
>  	 */
> -	struct perf_event_context	*parent_ctx;
> +	struct perf_event_context __rcu	*parent_ctx;
>  	u64				parent_gen;
>  	u64				generation;
>  	int				pin_count;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 2173c23c25b4..2a8c5670b254 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
>  static int context_equiv(struct perf_event_context *ctx1,
>  			 struct perf_event_context *ctx2)
>  {
> +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> +
>  	lockdep_assert_held(&ctx1->lock);
>  	lockdep_assert_held(&ctx2->lock);
>  
> +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);

Bah.

Why are you  fixing all this sparse crap and making the code worse?