linux-kernel - Re: [PATCH v3 3/7] perf: Simplify perf_event_free

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Message-ID: <1d9d8520-bb4d-4af3-a395-4a4e594e67e8@amd.com>
Date: Mon, 17 Mar 2025 12:19:07 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, lucas.demarchi@...el.com, linux-kernel@...r.kernel.org,
 acme@...nel.org, namhyung@...nel.org, mark.rutland@....com,
 alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com,
 adrian.hunter@...el.com, kan.liang@...ux.intel.com,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v3 3/7] perf: Simplify perf_event_free_task() wait

Hi Peter,

> @@ -1223,8 +1223,14 @@ static void put_ctx(struct perf_event_co
>  	if (refcount_dec_and_test(&ctx->refcount)) {
>  		if (ctx->parent_ctx)
>  			put_ctx(ctx->parent_ctx);
> -		if (ctx->task && ctx->task != TASK_TOMBSTONE)
> -			put_task_struct(ctx->task);
> +		if (ctx->task) {
> +			if (ctx->task == TASK_TOMBSTONE) {
> +				smp_mb(); /* pairs with wait_var_event() */
> +				wake_up_var(&ctx->refcount);

perf_event_free_task() waits on "ctx->refcount == 1". But moving
wake_up_var() under refcount_dec_and_test() will cause
perf_event_free_task() to wait indefinitely. Right? So, shouldn't
wake_up_var() be outside? something like:

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1281,15 +1281,14 @@ static void put_ctx(struct perf_event_context *ctx)
 	if (refcount_dec_and_test(&ctx->refcount)) {
 		if (ctx->parent_ctx)
 			put_ctx(ctx->parent_ctx);
-		if (ctx->task) {
-			if (ctx->task == TASK_TOMBSTONE) {
-				smp_mb(); /* pairs with wait_var_event() */
-				wake_up_var(&ctx->refcount);
-			} else {
-				put_task_struct(ctx->task);
-			}
-		}
+		if (ctx->task && ctx->task != TASK_TOMBSTONE)
+			put_task_struct(ctx->task);
 		call_rcu(&ctx->rcu_head, free_ctx);
+	} else {
+		if (ctx->task == TASK_TOMBSTONE) {
+			smp_mb(); /* pairs with wait_var_event() */
+			wake_up_var(&ctx->refcount);
+		}
 	}
 }

Thanks,
Ravi