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

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Message-ID: <20130307000824.GA5702@neilslaptop.think-freely.org>
Date:	Wed, 6 Mar 2013 19:08:24 -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 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

Best
Neil

> 
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