linux-kernel - Re: [PATCH v4] target: close target_put_sess_cmd() vs. core_tmr_abort

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Date:	Mon, 18 Mar 2013 23:58:03 -0400
From:	Jörn Engel <joern@...fs.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH v4] target: close target_put_sess_cmd() vs.
 core_tmr_abort_task() race

On Mon, 18 March 2013 22:09:54 -0700, Greg Kroah-Hartman wrote:
> On Mon, Mar 18, 2013 at 11:31:12PM -0400, Jörn Engel wrote:
> > On Mon, 18 March 2013 18:53:54 -0700, Greg Kroah-Hartman wrote:
> 
> > > And why not _irqstore() anymore?
> > 
> > Because I thought the resulting code would be horrible.  But going
> > through the excercise, it does seem half as bad as I feared.  In fact,
> > I rather like it now.
> 
> You changed the kref code too, does it work better now?

It compiles.  I don't have a good testcase, so the procedure is to
throw it into the test infrastructure and wait a week.

> > It is possible for one thread to to take se_sess->sess_cmd_lock in
> > core_tmr_abort_task() before taking a reference count on
> > se_cmd->cmd_kref, while another thread in target_put_sess_cmd() drops
> > se_cmd->cmd_kref before taking se_sess->sess_cmd_lock.
> > 
> > This introduces kref_put_spinlock_irqsave() and uses it in
> > target_put_sess_cmd() to close the race window.
> > 
> > Signed-off-by: Joern Engel <joern@...fs.org>
> > ---
> >  drivers/target/target_core_transport.c |    7 +++----
> >  include/linux/kref.h                   |   32 ++++++++++++++++++++++++++++++++
> >  2 files changed, 35 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> > index 04ec9cb..7e856b9 100644
> > --- a/drivers/target/target_core_transport.c
> > +++ b/drivers/target/target_core_transport.c
> > @@ -2203,13 +2203,11 @@ out:
> >  	return ret;
> >  }
> >  
> > -static void target_release_cmd_kref(struct kref *kref)
> > +static void target_release_cmd_kref(struct kref *kref, unsigned long flags)
> >  {
> >  	struct se_cmd *se_cmd = container_of(kref, struct se_cmd, cmd_kref);
> >  	struct se_session *se_sess = se_cmd->se_sess;
> > -	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
> 
> Why pass flags to a release function?
> 
> I don't think you can do that, but it's been a while since I looked at
> the spinlock code.

The alternative would be to call local_irq_restore(flags); from
kref_put_spinlock_irqsave() and not pass the flags.  Getting rid of
the extra parameter would be nice.  But I'm not sure I want to prove
that
	spin_unlock(lock);
	local_irq_restore(flags);
is the same as 
	spin_unlock_irqrestore(lock, flags);
on all architectures and with all combinations of CONFIG options.  I
think it should be, but I wouldn't bet half a cookie on it.

Jörn

--
He who knows that enough is enough will always have enough.
-- Lao Tsu
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