Re: [PATCH 1/3] srcu: Eliminate the requirement of SRCU_SIZE_WAIT_CALL

[Pingfan Liu <kernelfans@gmail.com>]

On Mon, Nov 28, 2022 at 08:40:22AM +0000, Zhang, Qiang1 wrote:
> >The state SRCU_SIZE_WAIT_CALL is only used in srcu_gp_start_if_needed().
> >And it is not needed. Because counter_wrap_check has guarantee that both
> >srcu_gp_seq_needed and srcu_gp_seq_needed_exp are not far behind
> >srcu_gp_seq and no false alarm will be raised by the statement in
> >srcu_gp_start_if_needed()
> >
> >  ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)
> >
> >As a result, once if SRCU_SIZE_WAIT_BARRIER is seen, the tree snp can be
> >used immediately, not need to wait for another srcu_gp_end() to update
> >the srcu_gp_seq_needed and srcu_gp_seq_needed_exp to avoid false alarm.
> >
> >Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> >Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> >Cc: "Paul E. McKenney" <paulmck@kernel.org>
> >Cc: Frederic Weisbecker <frederic@kernel.org>
> >Cc: Josh Triplett <josh@joshtriplett.org>
> >Cc: Steven Rostedt <rostedt@goodmis.org>
> >Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >Cc: "Zhang, Qiang1" <qiang1.zhang@intel.com>
> >To: rcu@vger.kernel.org
> >---
> > kernel/rcu/srcutree.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> >index 1c304fec89c0..fe0759d89c2d 100644
> >--- a/kernel/rcu/srcutree.c
> >+++ b/kernel/rcu/srcutree.c
> >@@ -1092,7 +1092,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > 	check_init_srcu_struct(ssp);
> > 	idx = srcu_read_lock(ssp);
> > 	ss_state = smp_load_acquire(&ssp->srcu_size_state);
> >-	if (ss_state < SRCU_SIZE_WAIT_CALL)
> >+	if (ss_state < SRCU_SIZE_WAIT_BARRIER)
> > 		sdp = per_cpu_ptr(ssp->sda, 0);
> 
> Hi Pingfan
> 
> should it be used get_boot_cpu_id() ?
> 

I am not sure which branch to work on.

Thanks,

	Pingfan

> Thanks
> Zqiang
> 
> > 	else
> > 		sdp = raw_cpu_ptr(ssp->sda);
> >-- 
> >2.31.1
> >