Re: [PATCH rcu 06/12] srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()

[Neeraj Upadhyay]

>  
>  /*
> - * Returns approximate total of the readers' ->srcu_lock_count[] values
> - * for the rank of per-CPU counters specified by idx.
> + * Computes approximate total of the readers' ->srcu_lock_count[] values
> + * for the rank of per-CPU counters specified by idx, and returns true if
> + * the caller did the proper barrier (gp), and if the count of the locks
> + * matches that of the unlocks passed in.
>   */
> -static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
> +static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, unsigned long unlocks)
>  {
>  	int cpu;
> +	unsigned long mask = 0;
>  	unsigned long sum = 0;
>  
>  	for_each_possible_cpu(cpu) {
>  		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
>  
>  		sum += atomic_long_read(&sdp->srcu_lock_count[idx]);
> +		if (IS_ENABLED(CONFIG_PROVE_RCU))
> +			mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
>  	}
> -	return sum;
> +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
> +		  "Mixed reader flavors for srcu_struct at %ps.\n", ssp);

I am trying to understand the (unlikely) case where synchronize_srcu() is done before any
srcu reader lock/unlock lite call is done. Can new SRCU readers fail to observe the
updates?


> +	if (mask & SRCU_READ_FLAVOR_LITE && !gp)
> +		return false;

So, srcu_readers_active_idx_check() can potentially return false for very long
time, until the CPU executing srcu_readers_active_idx_check() does
at least one read lock/unlock lite call?

> +	return sum == unlocks;
>  }
>  
>  /*
> @@ -473,6 +482,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>   */
>  static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
>  {
> +	bool did_gp = !!(raw_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE);

sda->srcu_reader_flavor is only set when CONFIG_PROVE_RCU is enabled. But we
need the reader flavor information for srcu lite variant to work. So, lite
variant does not work when CONFIG_PROVE_RCU is disabled. Am I missing something
obvious here?



- Neeraj

>  	unsigned long unlocks;
>  
>  	unlocks = srcu_readers_unlock_idx(ssp, idx);
> @@ -482,13 +492,16 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
>  	 * unlock is counted. Needs to be a smp_mb() as the read side may
>  	 * contain a read from a variable that is written to before the
>  	 * synchronize_srcu() in the write side. In this case smp_mb()s
> -	 * A and B act like the store buffering pattern.
> +	 * A and B (or X and Y) act like the store buffering pattern.
>  	 *
> -	 * This smp_mb() also pairs with smp_mb() C to prevent accesses
> -	 * after the synchronize_srcu() from being executed before the
> -	 * grace period ends.
> +	 * This smp_mb() also pairs with smp_mb() C (or, in the case of X,
> +	 * Z) to prevent accesses after the synchronize_srcu() from being
> +	 * executed before the grace period ends.
>  	 */
> -	smp_mb(); /* A */
> +	if (!did_gp)
> +		smp_mb(); /* A */
> +	else
> +		synchronize_rcu(); /* X */
>  
>  	/*
>  	 * If the locks are the same as the unlocks, then there must have
> @@ -546,7 +559,7 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
>  	 * which are unlikely to be configured with an address space fully
>  	 * populated with memory, at least not anytime soon.
>  	 */
> -	return srcu_readers_lock_idx(ssp, idx) == unlocks;
> +	return srcu_readers_lock_idx(ssp, idx, did_gp, unlocks);
>  }
>