On Fri, 16 Mar 2018, Peter Zijlstra wrote:
> On Mon, Mar 12, 2018 at 03:28:45PM +0100, Sebastian Andrzej Siewior wrote:
> > In -RT task_blocks_on_rt_mutex() may return with -EAGAIN due to
> > (->pi_blocked_on == PI_WAKEUP_INPROGRESS) before it added itself as a
> > waiter. In such a case we must not call remove_waiter() because without
> > a waiter it will trigger the BUG_ON() statement.
> >
> > This was initially reported by Yimin Deng. Thomas Gleixner fixed it then
> > with an explicit check for waiters before calling remove_waiter() with
> > the following note:
> >
> > | Guard it with rt_mutex_has_waiters(). This is a quick fix which is
> > | easy to backport. The proper fix is to have a central check in
> > | remove_waiter() so we can call it unconditionally.
> >
> > This is the suggested change.
> > Now that it is possible to call remove_waiter() unconditionally I also
> > remove that check from rt_mutex_slowlock().
> >
> > Link:
> > https://lkml.kernel.org/r/CAAh1qt=dcl9auxnxanp5bktipp3m+qj4yb+gdohhxpvfcxw...@mail.gmail.com
> > Reported-and-debugged-by: Yimin Deng <[email protected]>
> > Suggested-by: Thomas Gleixner <[email protected]>
> > Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
> > ---
> > kernel/locking/rtmutex.c | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > index 65cc0cb984e6..57d28d8f5280 100644
> > --- a/kernel/locking/rtmutex.c
> > +++ b/kernel/locking/rtmutex.c
> > @@ -1068,12 +1068,15 @@ static void mark_wakeup_next_waiter(struct
> > wake_q_head *wake_q,
> > static void remove_waiter(struct rt_mutex *lock,
> > struct rt_mutex_waiter *waiter)
> > {
> > - bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
>
> I'm a little confused, but isn't it easier to make rt_mutex_top_waiter()
> return NULL if there aren't in fact any waiters?
That works as well.
Thanks,
tglx
> diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
> index 68686b3ec3c1..70bcafc385c4 100644
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -52,11 +52,13 @@ static inline int rt_mutex_has_waiters(struct rt_mutex
> *lock)
> static inline struct rt_mutex_waiter *
> rt_mutex_top_waiter(struct rt_mutex *lock)
> {
> - struct rt_mutex_waiter *w;
> + struct rb_node *leftmost = rb_first_cached(&lock->waiters);
> + struct rt_mutex_waiter *w = NULL;
>
> - w = rb_entry(lock->waiters.rb_leftmost,
> - struct rt_mutex_waiter, tree_entry);
> - BUG_ON(w->lock != lock);
> + if (leftmost) {
> + w = rb_entry(leftmost, struct rt_mutex_waiter, tree_entry);
> + BUG_ON(w->lock != lock);
> + }
>
> return w;
> }
>
