Re: [PATCH rcu 2/3] srcu: Check for srcu_read_lock_lite() across all CPUs

[Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>]

On 11/12/2024 7:01 AM, Paul E. McKenney wrote:
> If srcu_read_lock_lite() is used on a given srcu_struct structure, then
> the grace-period processing must to synchronize_rcu() instead of smp_mb()

s/to/do/

> between the scans of the ->srcu_unlock_count[] and ->srcu_lock_count[]
> counters.  Currently, it does that by testing the SRCU_READ_FLAVOR_LITE
> bit of the ->srcu_reader_flavor mask, which works well.  But only if
> the CPU running that srcu_struct structure's grace period has previously
> executed srcu_read_lock_lite(), which might not be the case, especially
> just after that srcu_struct structure has been created and initialized.
> 
> This commit therefore updates the srcu_readers_unlock_idx() function
> to OR together the ->srcu_reader_flavor masks from all CPUs, and
> then make the srcu_readers_active_idx_check() function that test the
> SRCU_READ_FLAVOR_LITE bit in the resulting mask.
> 
> Note that the srcu_readers_unlock_idx() function is already scanning all
> the CPUs to sum up the ->srcu_unlock_count[] fields and that this is on
> the grace-period slow path, hence no concerns about the small amount of
> extra work.
> 
> Reported-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
> Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@amd.com/
> Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()")
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/rcu/srcutree.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 70979f294768c..5991381b44383 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -458,7 +458,7 @@ static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, uns
>   * Returns approximate total of the readers' ->srcu_unlock_count[] values
>   * for the rank of per-CPU counters specified by idx.
>   */
> -static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
> +static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx, unsigned long *rdm)
>  {
>  	int cpu;
>  	unsigned long mask = 0;
> @@ -468,11 +468,11 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>  		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
>  
>  		sum += atomic_long_read(&sdp->srcu_unlock_count[idx]);
> -		if (IS_ENABLED(CONFIG_PROVE_RCU))
> -			mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
> +		mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
>  	}
>  	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
>  		  "Mixed reader flavors for srcu_struct at %ps.\n", ssp);
> +	*rdm = mask;
>  	return sum;
>  }
>  
> @@ -482,10 +482,11 @@ 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);
> +	unsigned long rdm;
>  	unsigned long unlocks;
>  
> -	unlocks = srcu_readers_unlock_idx(ssp, idx);
> +	unlocks = srcu_readers_unlock_idx(ssp, idx, &rdm);
> +	bool did_gp = !!(rdm & SRCU_READ_FLAVOR_LITE);

Move "did_gp" declaration up?


Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>


-Neeraj