Re: [PATCH RFC] perf: fix find_swevent_head() RCU lockdep splat

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, May 13, 2010 at 10:48:58PM +0200, Frederic Weisbecker wrote:
> Paul, does that look good to you?
> I've only compile tested. But if it's fine for you, I'll test
> it for real and provide you a sane patch.
> 
> This version won't use more parameters than necessary in the
> off case.
> 
> 
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index a4fa381..d765e48 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -4066,19 +4066,36 @@ static inline u64 swevent_hash(u64 type, u32 event_id)
>  	return hash_64(val, SWEVENT_HLIST_BITS);
>  }
>  
> -static struct hlist_head *
> -find_swevent_head(struct perf_cpu_context *ctx, u64 type, u32 event_id)
> +static inline struct hlist_head *
> +__find_swevent_head(struct swevent_hlist *hlist, u64 type, u32 event_id)
>  {
>  	u64 hash;
> -	struct swevent_hlist *hlist;



+	if (!hlist)
+		return NULL;



>  
>  	hash = swevent_hash(type, event_id);
>  
> +	return &hlist->heads[hash];
> +}
> +
> +static inline struct hlist_head *
> +find_swevent_head_rcu(struct perf_cpu_context *ctx, u64 type, u32 event_id)
> +{
> +	struct swevent_hlist *hlist;
> +
>  	hlist = rcu_dereference(ctx->swevent_hlist);
> -	if (!hlist)
> -		return NULL;
>  
> -	return &hlist->heads[hash];
> +	return __find_swevent_head(hlist, type, event_id);
> +}
> +
> +static inline struct hlist_head *
> +find_swevent_head(struct perf_cpu_context *ctx, u64 type,
> +		  u32 event_id, struct perf_event *event)
> +{
> +	struct swevent_hlist *hlist;
> +
> +	hlist = rcu_dereference_check(ctx->swevent_hlist,
> +				      lockdep_is_held(&event->ctx->lock));
> +
> +	return __find_swevent_head(hlist, type, event_id);
>  }
>  
>  static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
> @@ -4095,7 +4112,7 @@ static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
>  
>  	rcu_read_lock();
>  
> -	head = find_swevent_head(cpuctx, type, event_id);
> +	head = find_swevent_head_rcu(cpuctx, type, event_id);
>  
>  	if (!head)
>  		goto end;
> @@ -4178,7 +4195,8 @@ static int perf_swevent_enable(struct perf_event *event)
>  		perf_swevent_set_period(event);
>  	}
>  
> -	head = find_swevent_head(cpuctx, event->attr.type, event->attr.config);
> +	head = find_swevent_head(cpuctx, event->attr.type,
> +				 event->attr.config, event);
>  	if (WARN_ON_ONCE(!head))
>  		return -EINVAL;
>  
> 
>