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

[Guenter Roeck]

Hi Steven,

On 1/27/26 07:22, Steven Rostedt wrote:

On Mon, 26 Jan 2026 17:32:35 -0800
Guenter Roeck <li...@roeck-us.net> wrote:

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

I figured there would be other locations. A while ago I had a patch to wrap
the checks in a "is_user_thread()" helper function[1], but Ingo had issues
with it. It seems now it's biting us in the butt and let's see if it would
help now. I modified it slightly.

[1] 
https://lore.kernel.org/linux-trace-kernel/20250425204313.616425...@goodmis.org/

If the below fixes it, I'll resend it, but now as a real bug fix.

Trying. So far I can no longer reproduce the problem with the patch below 
applied.
Obviously that doesn't mean that the problem is fixed, only that I can no longer
reproduce it. I'll keep trying with different platforms.

Guenter

-- Steve

diff --git a/include/linux/sched.h b/include/linux/sched.h
index da0133524d08..5f00b5ed0f3b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1776,6 +1776,11 @@ static __always_inline bool is_percpu_thread(void)
                (current->nr_cpus_allowed  == 1);
  }

+static __always_inline bool is_user_task(struct task_struct *task)

+{
+       return task->mm && !(task->flags & (PF_KTHREAD | PF_USER_WORKER));
+}
+
  /* Per-process atomic flags. */
  #define PFA_NO_NEW_PRIVS              0       /* May not gain new privileges. 
*/
  #define PFA_SPREAD_PAGE                       1       /* Spread page cache 
over cpuset */
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1f6589578703..9d24b6e0c91f 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -246,7 +246,7 @@ 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))
+                       if (!is_user_task(current))
                                goto exit_put;
                        regs = task_pt_regs(current);
                }
diff --git a/kernel/events/core.c b/kernel/events/core.c
index a0fa488bce84..8cca80094624 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7460,7 +7460,7 @@ static void perf_sample_regs_user(struct perf_regs 
*regs_user,
        if (user_mode(regs)) {
                regs_user->abi = perf_reg_abi(current);
                regs_user->regs = regs;
-       } else if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+       } else if (is_user_task(current)) {
                perf_get_regs_user(regs_user, regs);
        } else {
                regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
@@ -8100,7 +8100,7 @@ static u64 perf_virt_to_phys(u64 virt)
                 * Try IRQ-safe get_user_page_fast_only first.
                 * If failed, leave phys_addr as 0.
                 */
-               if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+               if (is_user_task(current)) {
                        struct page *p;

pagefault_disable();

@@ -8215,7 +8215,7 @@ perf_callchain(struct perf_event *event, struct pt_regs 
*regs)
  {
        bool kernel = !event->attr.exclude_callchain_kernel;
        bool user   = !event->attr.exclude_callchain_user &&
-               !(current->flags & (PF_KTHREAD | PF_USER_WORKER));
+               is_user_task(current);
        /* Disallow cross-task user callchains. */
        bool crosstask = event->ctx->task && event->ctx->task != current;
        bool defer_user = IS_ENABLED(CONFIG_UNWIND_USER) && user &&