From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
"x86@kernel.org" <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
syzkaller <syzkaller@googlegroups.com>,
Kostya Serebryany <kcc@google.com>,
Alexander Potapenko <glider@google.com>,
Sasha Levin <sasha.levin@oracle.com>
Subject: Re: lockdep WARNING in get_online_cpus
Date: Mon, 4 Apr 2016 11:01:43 +0200 [thread overview]
Message-ID: <20160404090143.GW3448@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CACT4Y+YO6DNEPB5BD7G4hw07XbYCZ=4v4QyR9su1jpOEQPNeEg@mail.gmail.com>
On Mon, Apr 04, 2016 at 10:19:05AM +0200, Dmitry Vyukov wrote:
>
> This happens in CONFIG_DEBUG_LOCKDEP code. Is it a bug in lockdep?
We hope not; but it is a new test.
So lockdep needs to check each current lock stack against the recorded
lock dependencies to see if we've gotten ourselves a cycle. Doing this
check is _expensive_.
So what lockdep does is it computes a hash for each lock stack and only
if we've not seen this hash before (actually truncated since we don't
have a full 64bit hashtable) do we go look for cycles.
The new check tries to detect hash-collisions in this cache. A collision
would result in not checking for cycles, even if we've not seen the
stack before.
You've managed to tickle this.
Now, last week I found some bugs in there, and Alfredo added a pretty
printer, so maybe try and add these patches to your testing?
git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/urgent
> But I always see at the same stack involving perf and jump_label...
So you have a simple reproducer? So that I can have a go at this.
next prev parent reply other threads:[~2016-04-04 9:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-02 13:02 lockdep WARNING in get_online_cpus Dmitry Vyukov
2016-04-04 8:19 ` Dmitry Vyukov
2016-04-04 9:01 ` Peter Zijlstra [this message]
2016-04-04 9:50 ` Dmitry Vyukov
2016-04-05 9:41 ` Peter Zijlstra
2016-04-05 9:44 ` Dmitry Vyukov
2016-04-05 11:18 ` Peter Zijlstra
2016-04-05 11:23 ` Dmitry Vyukov
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