From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@kernel.org>,
Byungchul Park <byungchul.park@lge.com>,
syzbot
<bot+e7353c7141ff7cbb718e4c888a14fa92de41ebaa@syzkaller.appspotmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dan Williams <dan.j.williams@intel.com>,
Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>,
jglisse@redhat.com, LKML <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, shli@fb.com, syzkaller-bugs@googlegroups.com,
Thomas Gleixner <tglx@linutronix.de>,
Vlastimil Babka <vbabka@suse.cz>,
ying.huang@intel.com, kernel-team@lge.com
Subject: Re: possible deadlock in lru_add_drain_all
Date: Tue, 31 Oct 2017 15:52:47 +0100 [thread overview]
Message-ID: <20171031145247.5kjbanjqged34lbp@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CACT4Y+Zi_Gqh1V7QHzUdRuYQAtNjyNU2awcPOHSQYw9TsCwEsw@mail.gmail.com>
On Tue, Oct 31, 2017 at 04:55:32PM +0300, Dmitry Vyukov wrote:
> I noticed that for a simple 2 lock deadlock lockdep prints only 2
> stacks.
3, one of which is useless :-)
For the AB-BA we print where we acquire A (#0) where we acquire B while
holding A #1 and then where we acquire B while holding A the unwind at
point of splat.
The #0 trace is useless.
> FWIW in user-space TSAN we print 4 stacks for such deadlocks,
> namely where A was locked, where B was locked under A, where B was
> locked, where A was locked under B. It makes it easier to figure out
> what happens. However, for this report it seems to be 8 stacks this
> way. So it's probably hard either way.
Right, its a question of performance and overhead I suppose. Lockdep
typically only saves a stack trace when it finds a new link.
So only when we find the AB relation do we save the stacktrace; which
reflects the location where we acquire B. But by that time we've lost
where it was we acquire A.
If we want to save those stacks; we have to save a stacktrace on _every_
lock acquire, simply because we never know ahead of time if there will
be a new link. Doing this is _expensive_.
Furthermore, the space into which we store stacktraces is limited;
since memory allocators use locks we can't very well use dynamic memory
for lockdep -- that would give recursive and robustness issues.
Also, its usually not too hard to find the site where we took A if we
know the site of AB.
next prev parent reply other threads:[~2017-10-31 14:53 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <089e0825eec8955c1f055c83d476@google.com>
2017-10-27 9:34 ` possible deadlock in lru_add_drain_all Michal Hocko
2017-10-27 9:44 ` Dmitry Vyukov
2017-10-27 9:47 ` Dmitry Vyukov
2017-10-27 13:42 ` Michal Hocko
2017-10-30 8:22 ` Michal Hocko
2017-10-30 10:09 ` Byungchul Park
2017-10-30 15:10 ` Peter Zijlstra
2017-10-30 15:22 ` Peter Zijlstra
2017-10-31 13:13 ` Michal Hocko
2017-10-31 13:51 ` Peter Zijlstra
2017-10-31 13:55 ` Dmitry Vyukov
2017-10-31 14:52 ` Peter Zijlstra [this message]
2017-10-31 14:58 ` Michal Hocko
2017-10-31 15:10 ` Peter Zijlstra
2017-11-01 8:59 ` Byungchul Park
2017-11-01 12:01 ` Peter Zijlstra
2017-11-01 23:54 ` Byungchul Park
2018-02-14 14:01 ` Dmitry Vyukov
2018-02-14 15:44 ` Michal Hocko
2018-02-14 15:57 ` Dmitry Vyukov
2017-10-31 15:25 ` Peter Zijlstra
2017-10-31 15:45 ` Michal Hocko
2017-10-31 16:30 ` Peter Zijlstra
2017-11-01 8:31 ` Byungchul Park
2017-10-30 10:26 ` Byungchul Park
2017-10-30 11:48 ` Michal Hocko
2017-10-27 11:27 ` Vlastimil Babka
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