Re: [PATCH 16/19] context_tracking: Convert state to atomic_t

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Mar 09, 2022 at 06:17:02PM +0100, nicolas saenz julienne wrote:
> On Wed, 2022-03-02 at 16:48 +0100, Frederic Weisbecker wrote:
> > Context tracking's state and dynticks counter are going to be merged
> > in a single field so that both updates can happen atomically and at the
> > same time. Prepare for that with converting the state into an atomic_t.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
> > Cc: Joel Fernandes <joel@joelfernandes.org>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Nicolas Saenz Julienne <nsaenz@kernel.org>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Cc: Xiongfeng Wang <wangxiongfeng2@huawei.com>
> > Cc: Yu Liao<liaoyu15@huawei.com>
> > Cc: Phil Auld <pauld@redhat.com>
> > Cc: Paul Gortmaker<paul.gortmaker@windriver.com>
> > Cc: Alex Belits <abelits@marvell.com>
> > ---
> >  static __always_inline bool context_tracking_in_user(void)
> >  {
> > -	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
> > +	return __ct_state() == CONTEXT_USER;
> >  }
> 
> I was wondering whether it'd make more sense to use ct_state() for extra safety
> vs preemption, but it turns out the function isn't being used at all.
> 
> I figure it'd be better to remove it altogether and leave ct_state() as the
> goto function for this sort of checks.

Ah even better!

> 
> >  #else
> >  static inline bool context_tracking_in_user(void) { return false; }
> > diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> > index de247e758767..69db43548768 100644
> > --- a/kernel/context_tracking.c
> > +++ b/kernel/context_tracking.c
> > @@ -337,6 +337,7 @@ static __always_inline void context_tracking_recursion_exit(void)
> >   */
> >  void noinstr __ct_user_enter(enum ctx_state state)
> >  {
> > +	struct context_tracking *ct = this_cpu_ptr(&context_tracking);
> 
> I wonder if there is any value to having __ct_state() take 'struct
> context_tracking *ct' as an argument to avoid a redundant this_cpu_ptr()...
> 
> >  	lockdep_assert_irqs_disabled();
> >  
> >  	/* Kernel threads aren't supposed to go to userspace */
> > @@ -345,8 +346,8 @@ void noinstr __ct_user_enter(enum ctx_state state)
> >  	if (!context_tracking_recursion_enter())
> >  		return;
> >  
> > -	if ( __this_cpu_read(context_tracking.state) != state) {
> > -		if (__this_cpu_read(context_tracking.active)) {
> > +	if (__ct_state() != state) {
> 
> ...here (and in __ct_user_exit()).

Hmm, I'll check that.

Thanks!