Re: [PATCH v4] target: close target_put_sess_cmd() vs. core_tmr_abort_task() race

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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:
> > On Mon, Mar 18, 2013 at 07:34:13PM -0400, Jörn Engel wrote:
> > > ---
> > >  drivers/target/target_core_transport.c |   17 +++++++++++------
> > >  include/linux/kref.h                   |   26 ++++++++++++++++++++++++++
> > >  2 files changed, 37 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> > > index 04ec9cb..b98c158 100644
> > > --- a/drivers/target/target_core_transport.c
> > > +++ b/drivers/target/target_core_transport.c
> > > @@ -2207,21 +2207,19 @@ static void target_release_cmd_kref(struct kref *kref)
> > >  {
> > >  	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);
> > >  	if (list_empty(&se_cmd->se_cmd_list)) {
> > > -		spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
> > > +		spin_unlock(&se_sess->sess_cmd_lock);
> > 
> > Wait, who has this locked?  You took out the call to spin_lock_* above.
> 
> The spinlock needs to be taken atomically wrt. the final kref_put.  So
> either the kref_put was not final and we don't end up calling
> target_release_cmd_kref() or it was and the spinlock is already taken
> when calling the release function.  You could call that the central
> bit of the patch. ;)

Ah, ok.

> > 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?



> 
> Jörn
> 
> --
> Happiness isn't having what you want, it's wanting what you have.
> -- unknown
> 
> 
> 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@logfs.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.

greg k-h