Re: [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter

[Neeraj Upadhyay]

Hi Paul,

On 11/17/2020 6:10 AM, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
>
> There is a need for a polling interface for SRCU grace periods.  This
> polling needs to distinguish between an SRCU instance being idle on the
> one hand or in the middle of a grace period on the other.  This commit
> therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from
> a defacto boolean to a free-running counter, using the bottom bit to
> indicate that a grace period is in progress.  The second-from-bottom
> bit is thus used as the index returned by srcu_read_lock().
>
> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@xxxxxxxxxxxxxx/
> Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>   include/linux/srcutiny.h | 4 ++--
>   kernel/rcu/srcutiny.c    | 5 +++--
>   2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> index 5a5a194..fed4a2d 100644
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -15,7 +15,7 @@
>   
>   struct srcu_struct {
>   	short srcu_lock_nesting[2];	/* srcu_read_lock() nesting depth. */
> -	short srcu_idx;			/* Current reader array element. */
> +	unsigned short srcu_idx;	/* Current reader array element in bit 0x2. */
>   	u8 srcu_gp_running;		/* GP workqueue running? */
>   	u8 srcu_gp_waiting;		/* GP waiting for readers? */
>   	struct swait_queue_head srcu_wq;
> @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp)
>   {
>   	int idx;
>   
> -	idx = READ_ONCE(ssp->srcu_idx);
> +	idx = (READ_ONCE(ssp->srcu_idx) & 0x2) / 2;

Should we use bit 0x2 of (READ_ONCE(ssp->srcu_idx) + 1) , if GP 
(srcu_drive_gp()) is in progress? Or am I missing something here?

idx = ((READ_ONCE(ssp->srcu_idx) +1) & 0x2) / 2;

Also, any reason for using divison instead of shift; something to
do with 16-bit srcu_idx which I am missing here?

Thanks
Neeraj

>   	WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
>   	return idx;
>   }
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 6208c1d..5598cf6 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp)
>   	ssp->srcu_cb_head = NULL;
>   	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
>   	local_irq_enable();
> -	idx = ssp->srcu_idx;
> -	WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
> +	idx = (ssp->srcu_idx & 0x2) / 2;
> +	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
>   	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
>   	swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
>   	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
> +	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
>   
>   	/* Invoke the callbacks we removed above. */
>   	while (lh) {

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