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

Greg Kroah-Hartman
2013-03-19T05:08:38Z

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