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Subject: Re: [PATCH] lockdep: Add debug printk() for downgrade_write()
 warning.
References: <1546771139-9349-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <162036ee-81f1-36b1-7658-15a5f98c7d29@i-love.sakura.ne.jp>
Date: Wed, 9 Jan 2019 20:47:43 +0900
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Andrew Morton <akpm@linux-foundation.org>, Linux-MM <linux-mm@kvack.org>, Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>

Hello, Peter.

We got two reports. Neither RWSEM_READER_OWNED nor RWSEM_ANONYMOUSLY_OWNED is set, and
(presumably) sem->owner == current is true, but count is -1. What does this mean?

https://syzkaller.appspot.com/text?tag=CrashLog&x=169dbb9b400000

[ 2580.337550][ T3645] mmap_sem: hlock->read=1 count=-4294967295 current=ffff888050e04140, owner=ffff888050e04140
[ 2580.353526][ T3645] ------------[ cut here ]------------
[ 2580.367859][ T3645] downgrading a read lock
[ 2580.367935][ T3645] WARNING: CPU: 1 PID: 3645 at kernel/locking/lockdep.c:3572 lock_downgrade+0x35d/0xbe0
[ 2580.382206][ T3645] Kernel panic - not syncing: panic_on_warn set ...

https://syzkaller.appspot.com/text?tag=CrashLog&x=1542da4f400000

[  386.342585][T16698] mmap_sem: hlock->read=1 count=-4294967295 current=ffff8880512ae180, owner=ffff8880512ae180
[  386.348586][T16698] ------------[ cut here ]------------
[  386.357203][T16698] downgrading a read lock
[  386.357294][T16698] WARNING: CPU: 1 PID: 16698 at kernel/locking/lockdep.c:3572 lock_downgrade+0x35d/0xbe0
[  386.372148][T16698] Kernel panic - not syncing: panic_on_warn set ...