linux-kernel - Re: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached

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Message-ID: <CAH+eYFBkm+D_N2tF8F_Pdk8qbVeTFcNefeJ6ROcDW3vLgsCGcw@mail.gmail.com>
Date:	Thu, 19 Jan 2012 01:13:21 +0530
From:	Rabin Vincent <rabin@....in>
To:	Chanho Min <chanho0207@...il.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached

On Mon, Jan 16, 2012 at 12:04, Chanho Min <chanho0207@...il.com> wrote:
>> But wait.. Rabin's patch actually fixes the NULL deference in
>> the call
>>
>>        trace_writeback_wake_forker_thread(bdi);
>>
>> The wakeup of the forker thread should be harmless.
>>
> The forker thread shoud not be woken up after the bdi is unregistered.
> Is this also Rabin's intention?
> Also, I'm not sure the wakeup of the forker thread is harmless as bellows.
> ..
> case FORK_THREAD:
> ..
>  task = kthread_create(bdi_writeback_thread, &bdi->wb,
>                      "flush-%s", dev_name(bdi->dev));

By the time the bdi->dev is made NULL, the bdi has been removed from the
bdi_list (in bdi_wb_shutdown()), so even if the forker thread is woken
up after that it will not find the bdi and won't get to this part of the
code.
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