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

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Message-ID: <20120116055354.GA15265@localhost>
Date:	Mon, 16 Jan 2012 13:53:54 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Chanho Min <chanho0207@...il.com>
Cc:	Rabin Vincent <rabin@....in>, 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 01:50:44PM +0800, Wu Fengguang wrote:
> On Mon, Jan 16, 2012 at 02:28:30PM +0900, Chanho Min wrote:
> > On Mon, Jan 16, 2012 at 11:53 AM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > On Sun, Jan 15, 2012 at 09:11:07PM +0530, Rabin Vincent wrote:
> > >> On Sun, Jan 15, 2012 at 08:58:53PM +0800, Wu Fengguang wrote:
> > >> > On Sun, Jan 15, 2012 at 03:58:43PM +0530, Rabin Vincent wrote:
> > >> > > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > >> > > index 71034f4..a39ad70 100644
> > >> > > --- a/mm/backing-dev.c
> > >> > > +++ b/mm/backing-dev.c
> > >> > > @@ -318,7 +318,7 @@ static void wakeup_timer_fn(unsigned long data)
> > >> > >   if (bdi->wb.task) {
> > >> > >           trace_writeback_wake_thread(bdi);
> > >> > >           wake_up_process(bdi->wb.task);
> > >> > > - } else {
> > >> > > + } else if (bdi->dev) {
> > >> > >           /*
> > >> > >            * When bdi tasks are inactive for long time, they are killed.
> > >> > >            * In this case we have to wake-up the forker thread which
> > >> > > @@ -584,6 +584,8 @@ EXPORT_SYMBOL(bdi_register_dev);
> > >> > >   */
> > >> > >  static void bdi_wb_shutdown(struct backing_dev_info *bdi)
> > >> > >  {
> > >> > > + struct task_struct *task = NULL;
> > 
> > Thanks, I fully understand.
> > In addition, Would not bdi_wakeup_flusher also be fixed?
> 
> Yes, indeed!

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.

Thanks,
Fengguang

> > >From dc7d4e86911c0a7ea35043485b04e8a09aa74ffd Mon Sep 17 00:00:00 2001
> > From: Chanho Min <chanho.min@....com>
> > Date: Mon, 16 Jan 2012 14:14:48 +0900
> > Subject: [PATCH] backing-dev: add for 'fix wakeup timer races with
> > bdi_unregister()'
> > 
> > Signed-off-by: Chanho Min <chanho.min@....com>
> > ---
> >  fs/fs-writeback.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index f855916..ee40833 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -97,7 +97,7 @@ static void bdi_wakeup_flusher(struct backing_dev_info *bdi)
> >  {
> >         if (bdi->wb.task) {
> >                 wake_up_process(bdi->wb.task);
> > -       } else {
> > +       } else if (bdi->dev) {
> >                 /*
> >                  * The bdi thread isn't there, wake up the forker thread which
> >                  * will create and run it.
> > -- 
> > 1.7.0.4
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