linux-kernel - Re: fs: GPF in locked_inode_to_wb_and_lock

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Message-ID: <CACT4Y+YSRq=BzA+rCNRX10=GBQZC84hc4gc6GjoGwCgoRz9HPQ@mail.gmail.com>
Date:	Thu, 21 Apr 2016 12:00:38 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: fs: GPF in locked_inode_to_wb_and_lock_list

On Thu, Apr 21, 2016 at 11:45 AM, Andrey Ryabinin
<ryabinin.a.a@...il.com> wrote:
> 2016-04-21 11:35 GMT+03:00 Dmitry Vyukov <dvyukov@...gle.com>:
>>
>> ffffffff818884dd:       48 8b 03                mov    (%rbx),%rax
>>
>> So whatever load "&wb->bdi->wb" produces is a NULL deref. (is it wb
>> that is NULL?)
>
> Yes it's NULL wb, because there is only one load:
>     mov    (%rbx),%rax        =>       rax = wb->bdi
>     add    $0x50,%rax         =>       rax = &bdi->wb


I bet that wb becomes NULL on the second iteration of the loop. The
loop loops in case of a race with another thread, so it would also
explain why it is difficult to reproduce.

Tejun, does it make any sense to you?