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From: Joel Fernandes <joel@joelfernandes.org>
To: dsterba@suse.cz,
	Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
	clm@fb.com, josef@toxicpanda.com, dsterba@suse.com,
	linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	paulmck@kernel.org, frextrite@gmail.com, linux@roeck-us.net
Subject: Re: [PATCH] fs: btrfs: block-group.c: Fix suspicious RCU usage warning
Date: Fri, 6 Mar 2020 14:53:23 -0500	[thread overview]
Message-ID: <20200306195323.GE60713@google.com> (raw)
In-Reply-To: <20200306152527.GH2902@twin.jikos.cz>

On Fri, Mar 06, 2020 at 04:25:27PM +0100, David Sterba wrote:
> On Fri, Mar 06, 2020 at 07:30:24PM +0530, Madhuparna Bhowmik wrote:
> > On Fri, Mar 06, 2020 at 03:16:53PM +0800, Qu Wenruo wrote:
> > > 
> > > 
> > > On 2020/3/6 下午2:52, madhuparnabhowmik10@gmail.com wrote:
> > > > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > > > 
> > > > The space_info list is rcu protected.
> > > > Hence, it should be traversed with rcu_read_lock held.
> > > > 
> > > > Warning:
> > > > [   29.104591] =============================
> > > > [   29.104756] WARNING: suspicious RCU usage
> > > > [   29.105046] 5.6.0-rc4-next-20200305 #1 Not tainted
> > > > [   29.105231] -----------------------------
> > > > [   29.105401] fs/btrfs/block-group.c:2011 RCU-list traversed in non-reader section!!
> > > > 
> > > > Reported-by: Guenter Roeck <linux@roeck-us.net>
> > > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > > > ---
> > > >  fs/btrfs/block-group.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> > > > index 404e050ce8ee..9cabeef66f5b 100644
> > > > --- a/fs/btrfs/block-group.c
> > > > +++ b/fs/btrfs/block-group.c
> > > > @@ -1987,6 +1987,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> > > 
> > > This function is only triggered at mount time, where no other rcu
> > > operation can happen.
> > >
> > Thanks Qu.
> > 
> > Joel and Paul, what should we do in this case?
> > Should we just pass cond = true or use list_for_each_entry instead?
> 
> I think we can afford to add rcu lock/unlock, even if it's not strictly
> necessary due to the single threaded context where the function is run.
> There are some lightweight operations inside and inc_block_group starts
> with two spin locks so there's nothing we'd be losing with disabled
> preemption from the caller.

I think use list_for_each_entry().

thanks,

 - Joel


WARNING: multiple messages have this Message-ID (diff)
From: Joel Fernandes <joel@joelfernandes.org>
To: dsterba@suse.cz,
	Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
	clm@fb.com, josef@toxicpanda.com, dsterba@suse.com,
	linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	paulmck@kernel.org, frextrite@gmail.com, linux@roeck-us.net
Subject: Re: [Linux-kernel-mentees] [PATCH] fs: btrfs: block-group.c: Fix suspicious RCU usage warning
Date: Fri, 6 Mar 2020 14:53:23 -0500	[thread overview]
Message-ID: <20200306195323.GE60713@google.com> (raw)
In-Reply-To: <20200306152527.GH2902@twin.jikos.cz>

On Fri, Mar 06, 2020 at 04:25:27PM +0100, David Sterba wrote:
> On Fri, Mar 06, 2020 at 07:30:24PM +0530, Madhuparna Bhowmik wrote:
> > On Fri, Mar 06, 2020 at 03:16:53PM +0800, Qu Wenruo wrote:
> > > 
> > > 
> > > On 2020/3/6 下午2:52, madhuparnabhowmik10@gmail.com wrote:
> > > > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > > > 
> > > > The space_info list is rcu protected.
> > > > Hence, it should be traversed with rcu_read_lock held.
> > > > 
> > > > Warning:
> > > > [   29.104591] =============================
> > > > [   29.104756] WARNING: suspicious RCU usage
> > > > [   29.105046] 5.6.0-rc4-next-20200305 #1 Not tainted
> > > > [   29.105231] -----------------------------
> > > > [   29.105401] fs/btrfs/block-group.c:2011 RCU-list traversed in non-reader section!!
> > > > 
> > > > Reported-by: Guenter Roeck <linux@roeck-us.net>
> > > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > > > ---
> > > >  fs/btrfs/block-group.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> > > > index 404e050ce8ee..9cabeef66f5b 100644
> > > > --- a/fs/btrfs/block-group.c
> > > > +++ b/fs/btrfs/block-group.c
> > > > @@ -1987,6 +1987,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> > > 
> > > This function is only triggered at mount time, where no other rcu
> > > operation can happen.
> > >
> > Thanks Qu.
> > 
> > Joel and Paul, what should we do in this case?
> > Should we just pass cond = true or use list_for_each_entry instead?
> 
> I think we can afford to add rcu lock/unlock, even if it's not strictly
> necessary due to the single threaded context where the function is run.
> There are some lightweight operations inside and inc_block_group starts
> with two spin locks so there's nothing we'd be losing with disabled
> preemption from the caller.

I think use list_for_each_entry().

thanks,

 - Joel

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  reply	other threads:[~2020-03-06 19:53 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-06  6:52 [PATCH] fs: btrfs: block-group.c: Fix suspicious RCU usage warning madhuparnabhowmik10
2020-03-06  6:52 ` [Linux-kernel-mentees] " madhuparnabhowmik10
2020-03-06  7:16 ` Qu Wenruo
2020-03-06  7:16   ` [Linux-kernel-mentees] " Qu Wenruo
2020-03-06 14:00   ` Madhuparna Bhowmik
2020-03-06 14:00     ` [Linux-kernel-mentees] " Madhuparna Bhowmik
2020-03-06 15:25     ` David Sterba
2020-03-06 15:25       ` [Linux-kernel-mentees] " David Sterba
2020-03-06 19:53       ` Joel Fernandes [this message]
2020-03-06 19:53         ` Joel Fernandes
2020-03-06 19:37 ` David Sterba
2020-03-06 19:37   ` [Linux-kernel-mentees] " David Sterba

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