Re: [RESEND][PATCH 3/5] perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 26 Jan 2026 12:05:53 -0500
Steven Rostedt <rostedt@kernel.org> wrote:

> I guess we need to also test for !current->mm because the flags set for an
> exiting task is done when we can still do callchains. Thus, the only way to
> know if it is safe to do a callchain when a task is exiting is via task->mm
> and not task->flags :-/

Can you test this patch?

-- Steve

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1f6589578703..c82d61d73bd8 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -246,7 +246,14 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 
 	if (user && !crosstask) {
 		if (!user_mode(regs)) {
-			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
+			/*
+			 * Testing current->mm is not enough as some kernel threads
+			 * may have one set. But testing the flags is not enough
+			 * either as this can be called after a user task
+			 * frees its mm just before it exits.
+			 */
+			if (!current->mm ||
+			    (current->flags & (PF_KTHREAD | PF_USER_WORKER)))
 				goto exit_put;
 			regs = task_pt_regs(current);
 		}