On Mon, 29 Mar 2021 at 16:27, Oleg Nesterov [off-list ref] wrote:

On 03/29, Peter Zijlstra wrote:

quoted

On Thu, Mar 25, 2021 at 09:14:39AM +0100, Marco Elver wrote:

quoted

@@ -6395,6 +6395,13 @@ static void perf_sigtrap(struct perf_event *event)
 {
    struct kernel_siginfo info;

+   /*
+    * This irq_work can race with an exiting task; bail out if sighand has
+    * already been released in release_task().
+    */
+   if (!current->sighand)
+           return;

This is racy. If "current" has already passed exit_notify(), current->parent
can do release_task() and destroy current->sighand right after the check.

quoted

Urgh.. I'm not entirely sure that check is correct, but I always forget
the rules with signal. It could be we ought to be testing PF_EXISTING
instead.

Agreed, PF_EXISTING check makes more sense in any case, the exiting task
can't receive the signal anyway.

So, per off-list discussion, it appears that I should ask to clarify:
PF_EXISTING or PF_EXITING?

It appears that PF_EXISTING is what's being suggested, whereas it has
not been mentioned anywhere, nor are its semantics clear. If it is not
simply the negation of PF_EXITING, what are its semantics? And why do
we need it in the case here (instead of something else that already
exists)?

Thanks,
-- Marco