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Date:   Tue, 25 Jan 2022 17:09:00 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        Baoquan He <bhe@redhat.com>, Vivek Goyal <vgoyal@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v2] proc/vmcore: fix possible deadlock on concurrent
 mmap and read
Message-Id: <20220125170900.472fdb649312e77a4a60d9da@linux-foundation.org>
In-Reply-To: <20220119193417.100385-1-david@redhat.com>
References: <20220119193417.100385-1-david@redhat.com>
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On Wed, 19 Jan 2022 20:34:17 +0100 David Hildenbrand <david@redhat.com> wrote:

> Lockdep noticed that there is chance for a deadlock if we have
> concurrent mmap, concurrent read, and the addition/removal of a
> callback.
> 
> As nicely explained by Boqun:
> 
> "
> Lockdep warned about the above sequences because rw_semaphore is a fair
> read-write lock, and the following can cause a deadlock:
> 
> 	TASK 1			TASK 2		TASK 3
> 	======			======		======
> 	down_write(mmap_lock);
> 				down_read(vmcore_cb_rwsem)
> 						down_write(vmcore_cb_rwsem); // blocked
> 	down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
> 				down_read(mmap_lock); // blocked

I'm wondering about cc:stable.  It's hard to believe that this is
likely to be observed in real life.  But the ongoing reports of lockdep
splats will be irritating.