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

[Guenter Roeck]

On 1/26/26 09:18, Steven Rostedt wrote:

On Mon, 26 Jan 2026 12:05:53 -0500
Steven Rostedt <rost...@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?

Still crashing, though not as often and with a slightly different backtrace.
I added the backtrace to the bug report @ Google.

Guenter

-- 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);
                }