linux-kernel - Re: WARN_ONCE in arch/x86/kernel/hw

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Message-ID: <20130528170048.GA26906@redhat.com>
Date:	Tue, 28 May 2013 19:00:48 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	trinity@...r.kernel.org,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: WARN_ONCE in arch/x86/kernel/hw_breakpoint.c

Well. I am not familiar with this code, and when I tried to read it
I feel I will be never able to understand it ;)

On 05/20, Vince Weaver wrote:
>
> on 3.10-rc1 with the trinity fuzzer patched to exercise the
> perf_event_open() syscall I am triggering this WARN_ONCE:
>
> [   75.864822] ------------[ cut here ]------------
> [   75.864830] WARNING: at arch/x86/kernel/hw_breakpoint.c:121 arch_install_hw_breakpoint+0x5b/0xcb()
...
> [   75.864916]  [<ffffffff81006fff>] ? arch_install_hw_breakpoint+0x5b/0xcb
> [   75.864919]  [<ffffffff810ab5a1>] ? event_sched_in+0x68/0x11c

I am wondering if we should check attr->pinned before WARN_ONCE...

But it seems that hw_breakpoint.c is buggy anyway.

Suppose that attr.task != NULL and event->cpu = -1.

__reserve_bp_slot() tries to calculate slots.pinned and calls
fetch_bp_busy_slots().

In this case fetch_bp_busy_slots() does

	for_each_online_cpu(cpu)
		...
		nr += task_bp_pinned(cpu, bp, type);

And task_bp_pinned() (in particular) checks cpu == event->cpu,
this will be never true.

IOW, it seems that __reserve_bp_slot(task, cpu => -1) always
succeeds because task_bp_pinned() returns 0 and thus we can
create more than HWP_NUM breakpoints. Much more ;)

As for _create, I guess we probably need something like

--- x/kernel/events/hw_breakpoint.c
+++ x/kernel/events/hw_breakpoint.c
@@ -156,7 +156,7 @@ fetch_bp_busy_slots(struct bp_busy_slots
 		if (!tsk)
 			nr += max_task_bp_pinned(cpu, type);
 		else
-			nr += task_bp_pinned(cpu, bp, type);
+			nr += task_bp_pinned(-1, bp, type);
 
 		if (nr > slots->pinned)
 			slots->pinned = nr;

But I simply can't understand toggle_bp_task_slot()->task_bp_pinned().

Oleg.

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