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

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Date:	Mon, 11 Mar 2013 12:39:14 +0100
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Veaceslav Falico <darkmag@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	amwang@...hat.com
Subject: Re: [PATCH] netconsole: release the spinlock before
 __netpoll_cleanup()

On Mon, Mar 11, 2013 at 07:30:24AM -0400, Neil Horman wrote:
>On Mon, Mar 11, 2013 at 11:08:02AM +0100, Veaceslav Falico wrote:
>> On Sun, Mar 10, 2013 at 4:25 PM, Veaceslav Falico <vfalico@...hat.com> wrote:
>> > On Thu, Mar 07, 2013 at 04:14:38PM -0500, David Miller wrote:
>> >>
>> ...snip...
>> >> Quite a bit of email corruption of this patch.
>> >
>> >
>> > Sorry, somehow messed it.
>> >
>> >
>> >>
>> >> Also, this code block is probably too deeply indented to be sane,
>> >> consider creating a small helper function to call instead.
>> >
>> >
>> > It gets quite ugly if I try to move it to another function. However, maybe
>> > something like that will work - it's effectively the same code, just that
>> > I've moved the long part out of the if () { } block. Looks a lot more
>> > readable, though one line still breaks 80chars limit. I've reworked the
>> > subject/commit message too.
>> >
>> > Subject: [PATCH] netconsole: don't call __netpoll_cleanup() while atomic
>> >
>> > __netpoll_cleanup() is called in netconsole_netdev_event() while holding a
>> > spinlock. Release/acquire the spinlock before/after it and restart the
>> >
>> > loop.
>> >
>> > Signed-off-by: Veaceslav Falico <vfalico@...hat.com>
>> > ---
>> >    drivers/net/netconsole.c |   22 +++++++++++++++-------
>> >    1 files changed, 15 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
>> > index 37add21..38eaa8c 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) {
>> > @@ -679,14 +680,21 @@ static int netconsole_netdev_event(struct
>> > notifier_block *this,
>> >                                 /*
>> >
>> >                                  * rtnl_lock already held
>> >                                  */
>> > -                               if (nt->np.dev) {
>> > -                                       __netpoll_cleanup(&nt->np);
>> > -                                       dev_put(nt->np.dev);
>> > -                                       nt->np.dev = NULL;
>> > +                               if (!nt->np.dev) {
>> > +                                       nt->enabled = 0;
>> > +                                       stopped = true;
>> > +                                       break;
>> >                                 }
>> > -                               nt->enabled = 0;
>> > -                               stopped = true;
>> > -                               break;
>> > +                               /*
>> > +                                * we might sleep in __netpoll_cleanup()
>> > +                                */
>> > +                               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;
>> >                         }
>> >                 }
>> >                 netconsole_target_put(nt);
>> > --
>> > 1.7.1
>>
>> Self-NAK this patch, I've triggered another kernel panic with it. Will
>> send another one shortly. Basicly, the whole if (!nt->np.dev) is not
>> needed and nt->enabled=0 should always be set, otherwise we
>> end up with nt->np.dev == NULL and nt->enabled == 1, thus
>> triggering panics in places like write_msg(), where it verifies only
>> if the nt->enabled is true.
>>
>Yup, I think you want to make the nt->enabled and stopped statements
>unconditional, and precedede the whole block with a if(!nt->np.dev) { continue;}
>statement.

I don't see why the statement is needed at all, if we verify it beforehand:

669         list_for_each_entry(nt, &target_list, list) {
670                 netconsole_target_get(nt);
671                 if (nt->np.dev == dev) {

so that effectively it can be null only in case the dev, delivered via
netconsole_netdev_event() is null, which is a bug by itself. Or am I
missing something?

>Neil
>
>> --
>> Best regards,
>> Veaceslav Falico
>>
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