Re: [PATCH v1 06/11] rcu: Enable RCU callbacks to benefit from expedited grace periods

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Jun 24, 2026 at 06:23:48AM -0700, Puranjay Mohan a écrit :
> Currently, RCU callbacks only track normal grace-period sequence
> numbers.  This means callbacks must wait for normal grace periods to
> complete even when expedited grace periods have already elapsed.
> 
> Use the full struct rcu_gp_seq (which tracks both the normal and
> expedited grace-period sequences) throughout the callback
> infrastructure.
> 
> rcu_segcblist_advance() now checks both normal and expedited GP
> completion via poll_state_synchronize_rcu_full(), and becomes
> parameterless since it reads the grace-period state internally.
> rcu_segcblist_accelerate() stores the full state (both sequences)
> instead of just the normal one.  rcu_accelerate_cbs() and
> rcu_accelerate_cbs_unlocked() use get_state_synchronize_rcu_full() to
> capture both sequences, and the NOCB advance checks use
> poll_state_synchronize_rcu_full() instead of comparing only the normal
> sequence.
> 
> srcu_segcblist_advance() becomes a standalone implementation because it
> compares SRCU sequences directly and cannot use
> poll_state_synchronize_rcu_full(), which reads RCU-specific globals.
> srcu_segcblist_accelerate() sets the ->exp field to
> RCU_GET_STATE_NOT_TRACKED so that poll_state_synchronize_rcu_full()
> compares only ->norm and ignores ->exp.
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  kernel/rcu/rcu_segcblist.c | 30 +++++++++++++++++++++++-------
>  kernel/rcu/rcu_segcblist.h |  2 +-
>  kernel/rcu/tree.c          |  9 +++------
>  kernel/rcu/tree_nocb.h     | 33 +++++++++++++++++++++++----------
>  4 files changed, 50 insertions(+), 24 deletions(-)
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 4e3dfe42bc097..cf8951d33e767 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -12,6 +12,7 @@
>  #include <linux/kernel.h>
>  #include <linux/types.h>
>  
> +#include "rcu.h"
>  #include "rcu_segcblist.h"
>  
>  /* Initialize simple callback list. */
> @@ -494,9 +495,9 @@ static void rcu_segcblist_advance_compact(struct rcu_segcblist *rsclp, int i)
>  
>  /*
>   * Advance the callbacks in the specified rcu_segcblist structure based
> - * on the current value passed in for the grace-period counter.
> + * on the current value of the grace-period counter.
>   */
> -void rcu_segcblist_advance(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp)
> +void rcu_segcblist_advance(struct rcu_segcblist *rsclp)
>  {
>  	int i;
>  
> @@ -509,7 +510,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp)
>  	 * are ready to invoke, and put them into the RCU_DONE_TAIL segment.
>  	 */
>  	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
> -		if (ULONG_CMP_LT(gsp->norm, rsclp->gp_seq[i].norm))
> +		if (!poll_state_synchronize_rcu_full(&rsclp->gp_seq[i]))

So after more careful review, the smp_mb() at the end of a successful
poll_state_synchronize_rcu_full() is necessary here because the current locking
is not enough to make sure we synchronize against the end of the grace period.

But what about the smp_mb() at the beginning? Paul what is the point of this one
already? It advertizes to pair with the smp_mb() on root cleanup but what
exactly is to be ordered here? Why does gp cleanup need to synchronize with
failing poll_state_synchronize_rcu_full() ? The smp_mb() before rcu_seq_snap()
in get_state_synchronize_rcu_full() should already synchronize the accesses
before that call against the beginning of the grace period.

If we keep all these barriers around and both RCU_WAIT_TAIL and RCU_NEXT_READY
need to be advanced, that makes 4 smp_mb() calls.

> @@ -637,14 +638,29 @@ void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
>  
>  void srcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
>  {
> -	struct rcu_gp_seq gs = { .norm = seq };
> +	int i;
> +
> +	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
> +	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
> +		return;
> +
> +	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
> +		if (ULONG_CMP_LT(seq, rsclp->gp_seq[i].norm))
> +			break;

Why not use the same API here and consolidate the code? ->exp is RCU_GET_STATE_NOT_TRACKED so it's
harmless?

> @@ -1164,7 +1164,7 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
>  	 * accelerating callback invocation to an earlier grace-period
>  	 * number.
>  	 */
> -	gs.norm = rcu_seq_snap(&rcu_state.gp_seq);
> +	get_state_synchronize_rcu_full(&gs);

I have similar concerns about the three smp_mb() in
get_state_synchronize_rcu_full(). It could be just two (rcu_seq_snap()
has a barrier that could be just one). Not sure if that matters but,
just wanted to point that.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs