From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io1-f72.google.com (mail-io1-f72.google.com [209.85.166.72]) by kanga.kvack.org (Postfix) with ESMTP id 0BCA68E0038 for ; Wed, 9 Jan 2019 09:19:18 -0500 (EST) Received: by mail-io1-f72.google.com with SMTP id a12so6491256iok.8 for ; Wed, 09 Jan 2019 06:19:18 -0800 (PST) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id 7si2570087itm.109.2019.01.09.06.19.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 09 Jan 2019 06:19:16 -0800 (PST) Subject: Re: WARNING: locking bug in lock_downgrade References: <00000000000043ae20057b974f14@google.com> <69273d51-c129-6b0f-35eb-d98655476ff9@redhat.com> From: Tetsuo Handa Message-ID: Date: Wed, 9 Jan 2019 23:18:44 +0900 MIME-Version: 1.0 In-Reply-To: <69273d51-c129-6b0f-35eb-d98655476ff9@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: peterz@infradead.org, "mingo@redhat.com" Cc: Waiman Long , Yang Shi , syzbot , akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux@dominikbrodowski.net, mhocko@suse.com, rientjes@google.com, syzkaller-bugs@googlegroups.com, vbabka@suse.cz, boqun.feng@gmail.com On 2018/12/14 4:46, Waiman Long wrote: > On 12/12/2018 08:14 PM, Yang Shi wrote: >> By looking into lockdep code, I'm not sure if lockdep may get confused >> by such sequence or not? >> >> >> Any hint is appreciated. >> >> >> Regards, >> >> Yang > > The warning was printed because hlock->read was set when doing the > downgrade_write(). So it is either downgrade_write() was called a second > time or a read lock was held originally. It is hard to tell what is the > root cause without a reproducer. > > Cheers, > Longman > Comparing with output from struct rw_semaphore *sem = ¤t->mm->mmap_sem; down_write(sem); pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); /* mmap_sem: count=-4294967295 current=ffff88813095ca80, owner=ffff88813095ca80 */ downgrade_write(sem); pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); /* mmap_sem: count=1 current=ffff88813095ca80, owner=ffff88813095ca83 */ up_read(sem); pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); /* mmap_sem: count=0 current=ffff88813095ca80, owner=0000000000000003 */ what we got with debug printk() patch 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 ... indicates that lockdep is saying that "current->mm->mmap_sem is held for read" while "struct rw_semaphore" is saying that "current->mm->mmap_sem is held for write". Something made lockdep confused. Possibly a lockdep bug.