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Date: Mon, 24 Oct 2016 13:15:27 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Ni, BaoleX" <baolex.ni@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
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        "alexander.shishkin@linux.intel.com" 
        <alexander.shishkin@linux.intel.com>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>
Subject: Re: hit a KASan bug related to Perf during stress test
Message-ID: <20161024111526.GA13509@redhat.com>
References: <318B87A793BE164187D8851D6CE09D64371C8811@shsmsx102.ccr.corp.intel.com> <20161024095341.GF3102@twins.programming.kicks-ass.net>
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On 10/24, Peter Zijlstra wrote:
>
> > [32738.867020]  [<ffffffff810d9975>] task_tgid_nr_ns+0x35/0xb0
>
> So here we did: perf_event_[pt]id(event, current);
>
> How can _current_ not be valid anymore?

...

> > [32739.040207]  [<ffffffff81135a4c>] __call_rcu+0x12c/0x450
>
> And while we just called release_task(), that call_rcu() should still be
> pending at this point,

Yes, current is still valid.

But nothing protects current->group_leader or parent/real_parent, they
can point to the exited/freed task. We really need to nullify them in
__unhash_process() to catch the problems like this, I wanted to do this
many times...

So you simply can't know your tgid or even tid after release_task() calls
__unhash_process(). Actually after exit_notify() unless the exiting task
autoreaps itself.

How about the trivial fix below?

Oleg.

--- x/kernel/events/core.c
+++ x/kernel/events/core.c
@@ -1257,7 +1257,7 @@ static u32 perf_event_pid(struct perf_ev
 	if (event->parent)
 		event = event->parent;
 
-	return task_tgid_nr_ns(p, event->ns);
+	return pid_alive(p) ? task_tgid_nr_ns(p, event->ns) : 0;
 }
 
 static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)