Re: [PATCH v1 4/5] seqlock: seqcount_LOCKTYPE_t: Introduce PREEMPT_RT support

peterz
2020-08-28T08:58:01Z

On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> +/*
> + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> + * associated lock does not implicitly disable preemption.
> + *
> + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> + */
> +#define __seq_enforce_preemption_protection(s) \
> + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))

Hurph, so basically you want to make __seqcount_lock_preemptible()
return true PREEMPT_RT ? Should we then not muck about with the propery
instead of this?

ISTR I had something like the below, would that not be the same but much
clearer ?

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 300cbf312546..3b5ad026ddfb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -211,11 +211,13 @@ static inline void __seqcount_assert(seqcount_t *s)
lockdep_assert_preemption_disabled();
}

-SEQCOUNT_LOCKTYPE(raw_spinlock_t, raw_spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t, spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t, rwlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex, mutex, true, s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
+#define __PREEMPT_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKTYPE(raw_spinlock_t, raw_spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(spinlock_t, spinlock, __PREEMPT_RT, s->lock)
+SEQCOUNT_LOCKTYPE(rwlock_t, rwlock, __PREEMPT_RT, s->lock)
+SEQCOUNT_LOCKTYPE(struct mutex, mutex, true, s->lock)
+SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)

/*
* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t