linux-kernel - Re: [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT

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Date:   Tue, 8 Sep 2020 13:45:20 +0200
From:   peterz@...radead.org
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce
 PREEMPT_RT support

On Fri, Sep 04, 2020 at 05:32:30PM +0200, Ahmed S. Darwish wrote:
> @@ -406,13 +443,20 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
>  	return __read_seqcount_t_retry(s, start);
>  }
>  
> +/*
> + * Enforce non-preemptibility for all seqcount_LOCKNAME_t writers. Don't
> + * do it for PREEMPT_RT, for the reasons outlined at __SEQ_LOCK().
> + */
> +#define __seq_enforce_writer_non_preemptibility(s)			\
> +	(!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
> +
>  /**
>   * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
>   * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
>   */
>  #define raw_write_seqcount_begin(s)					\
>  do {									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	raw_write_seqcount_t_begin(__seqcount_ptr(s));			\
> @@ -433,7 +477,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
>  do {									\
>  	raw_write_seqcount_t_end(__seqcount_ptr(s));			\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_enable();					\
>  } while (0)
>  
> @@ -456,7 +500,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
>  do {									\
>  	__seqcount_assert_lock_held(s);					\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass);	\
> @@ -483,7 +527,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
>  do {									\
>  	__seqcount_assert_lock_held(s);					\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	write_seqcount_t_begin(__seqcount_ptr(s));			\
> @@ -504,7 +548,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
>  do {									\
>  	write_seqcount_t_end(__seqcount_ptr(s));			\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_enable();					\
>  } while (0)

I've replaced the above with the below, afaict there were no users of
__seqcount_lock_preemptible() left.

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -228,7 +228,11 @@ __seqprop_##lockname##_sequence(const se
 static __always_inline bool						\
 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
 {									\
-	return preemptible;						\
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))				\
+		return preemptible;					\
+									\
+	/* PREEMPT_RT relies on the above LOCK+UNLOCK */		\
+	return false;							\
 }									\
 									\
 static __always_inline void						\