netdev - Re: [PATCH] netconsole: release the spinlock before __netpoll

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Message-ID: <20130307144355.GA2464@neilslaptop.think-freely.org>
Date:	Thu, 7 Mar 2013 09:43:55 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Veaceslav Falico <vfalico@...hat.com>
Cc:	netdev@...r.kernel.org, amwang@...hat.com, davem@...emloft.net
Subject: Re: [PATCH] netconsole: release the spinlock before
 __netpoll_cleanup()

On Thu, Mar 07, 2013 at 11:03:25AM +0100, Veaceslav Falico wrote:
> On Wed, Mar 06, 2013 at 07:08:24PM -0500, Neil Horman wrote:
> >On Wed, Mar 06, 2013 at 03:46:43PM +0100, Veaceslav Falico wrote:
> >>Commit 3335f0ca130c201f8680e97f63612053fbc16e22 removed spinlock unlocking
> >>before __netpoll_cleanup() in netconsole_netdev_event(), however we still
> >>might sleep in __netpoll_cleanup() - via synchronize_srcu(). Revert it and
> >>add a comment.
> >>
> >>Signed-off-by: Veaceslav Falico <vfalico@...hat.com>
> >>---
> >> drivers/net/netconsole.c |   10 ++++++++++
> >> 1 files changed, 10 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
> >>index 37add21..dd62b4c 100644
> >>--- a/drivers/net/netconsole.c
> >>+++ b/drivers/net/netconsole.c
> >>@@ -680,7 +680,17 @@ static int netconsole_netdev_event(struct notifier_block *this,
> >> 				 * rtnl_lock already held
> >> 				 */
> >> 				if (nt->np.dev) {
> >>+					/*
> >>+					 * we still might sleep in
> >>+					 * __netpoll_cleanup(), so release
> >>+					 * the lock
> >>+					 */
> >>+					spin_unlock_irqrestore(
> >>+							      &target_list_lock,
> >>+							      flags);
> >> 					__netpoll_cleanup(&nt->np);
> >>+					spin_lock_irqsave(&target_list_lock,
> >>+							  flags);
> >> 					dev_put(nt->np.dev);
> >> 					nt->np.dev = NULL;
> >> 				}
> >>--
> >>1.7.1
> >>
> >Thanks for noticing this Vaeceslav, but you can't just drop and re-acquire the
> >lock like this, as it protect the list_for_each_entry loop that you're in.  You
> >can drop the lock in the above if clause, but then, after the nt->np.dev = NULL,
> >go back an re-aquire the lock, and start the for loop.  I thought we had already
> >done this for some other purpose in this code using a label and a goto, but I
> >suppose I was mistaken
> 
> You're right, I somehow missed that restart goto, which was removed
> earlier. Does that feel right (I've also added back the
> netconsole_target_put()):
> 
> Subject: [PATCH] netconsole: release the spinlock before __netpoll_cleanup()
> 
> __netpoll_cleanup() might sleep in synchronize_srcu(), which was added to
> avoid race in another situation, so we can't call it with the spinlock
> target_list_lock held.
> 
> Add spin_unlock/lock before/after it and restart the loop.
> 
> Signed-off-by: Veaceslav Falico <vfalico@...hat.com>
> ---
>  drivers/net/netconsole.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
> index 37add21..267c26b 100644
> --- a/drivers/net/netconsole.c
> +++ b/drivers/net/netconsole.c
> @@ -666,6 +666,7 @@ static int netconsole_netdev_event(struct notifier_block *this,
>  		goto done;
>  	spin_lock_irqsave(&target_list_lock, flags);
> +restart:
>  	list_for_each_entry(nt, &target_list, list) {
>  		netconsole_target_get(nt);
>  		if (nt->np.dev == dev) {
> @@ -680,9 +681,21 @@ static int netconsole_netdev_event(struct notifier_block *this,
>  				 * rtnl_lock already held
>  				 */
>  				if (nt->np.dev) {
> +					/*
> +					 * we still might sleep in
> +					 * __netpoll_cleanup(), so release
> +					 * the lock and restart
> +					 */
> +					spin_unlock_irqrestore(
> +							      &target_list_lock,
> +							      flags);
>  					__netpoll_cleanup(&nt->np);
> +					spin_lock_irqsave(&target_list_lock,
> +							  flags);
>  					dev_put(nt->np.dev);
>  					nt->np.dev = NULL;
> +					netconsole_target_put(nt);
> +					goto restart;
>  				}
>  				nt->enabled = 0;
>  				stopped = true;
> -- 
> 1.7.1
> 
Yes, this looks better, thank you.
Acked-by: Neil Horman <nhorman@...driver.com>

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