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Date:   Wed, 30 Sep 2020 11:16:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>,
        g@hirez.programming.kicks-ass.net
Cc:     Qian Cai <cai@redhat.com>, Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: lockdep null-ptr-deref
Message-ID: <20200930091610.GV2628@hirez.programming.kicks-ass.net>
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On Wed, Sep 30, 2020 at 07:08:23AM +0800, Boqun Feng wrote:
> I think there are two problems here:
> 
> 1) the "(null)" means we don't have the "usage_str" for a usage bit,
> which I think is the LOCK_USED_READ bit introduced by Peter at
> 23870f122768 ('locking/lockdep: Fix "USED" <- "IN-NMI" inversions').
> 
> 2) the next null-ptr-deref, and I think this is also caused by
> LOCK_USED_READ bit, because in the loop inside
> print_lock_class_header(), we iterate from 0 to LOCK_USAGE_STATES (which
> is 4*2 + 3), however the class->usage_traces[] only has
> XXX_LOCK_USAGE_STATES (which is 4*2 + 1) elements, so if we have
> LOCK_USED_READ bit set in ->usage_mask, we will try to access an element
> out of the ->usage_traces[] array.
> 
> Probably the following helps? And another possible fix is to enlarge the
> ->usage_trace[] array and record the call trace of LOCK_READ_USED.

Urgh.. yeah, I wanted to avoid saving that trace; it's pretty useless :/
The existing USED trace is already mostly pointless, the consistent
thing would be to remove both but that might be too radical.

But you're right in that I made a right mess of it. Not sure what's
best here.

Let me have a play.