RE: [PATCH 1/2] srcu: Remove needless rcu_seq_done() check while holding read lock

["Zhang, Qiang1" <qiang1.zhang@intel.com>]

>
>As the code changes, now, srcu_gp_start_if_needed() holds the srcu read
>lock, gets the gp_seq snap and call srcu_funnel_gp_start() passing that
>snap value.
>
>As the rcu_seq_snap() promises "a full grace period has elapsed since
>the current time".  In srcu_funnel_gp_start(), the statement
>  rcu_seq_done(&ssp->srcu_gp_seq, s)
>always return false.

Hi Pingfan

Please correct me if I understand your commit message incorrectly. because the srcu_gp_start_if_needed() is
protected  by srcu read lock,  and the  rcu_seq_snap()  seq is a  at the end of the current or next srcu grace period, 
so as long as we are still in the srcu_gp_start_if_needed(), it also means that we are in SRCU critical section.
the current or next srcu grace period will not end, it also means that we can not invoke rcu_seq_end() to update 
'ssp->secu_gp_seq' , so the rcu_seq_done(&ssp->srcu_gp_seq, s) is always return false.

Thanks
Zqiang

>
>The same condition stands for srcu_funnel_exp_start(). Hence removing
>all the checks of rcu_seq_done().
>
>Test info:
>  Running "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10h --configs 18*SRCU-P"
>without any failure.
>
>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>
To: rcu@vger.kernel.org
---
 kernel/rcu/srcutree.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 1c304fec89c0..7df59fc8073e 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -851,8 +851,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
 	if (snp)
 		for (; snp != NULL; snp = snp->srcu_parent) {
 			sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp);
-			if (rcu_seq_done(&ssp->srcu_gp_seq, s) ||
-			    (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
+			if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s))
 				return;
 			spin_lock_irqsave_rcu_node(snp, flags);
 			sgsne = snp->srcu_gp_seq_needed_exp;
@@ -878,6 +877,9 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
  *
  * Note that this function also does the work of srcu_funnel_exp_start(),
  * in some cases by directly invoking it.
+ *
+ * The srcu read lock should be hold around this function. And s is a seq snap
+ * after holding that lock.
  */
 static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 				 unsigned long s, bool do_norm)
@@ -898,8 +900,6 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 	if (snp_leaf)
 		/* Each pass through the loop does one level of the srcu_node tree. */
 		for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
-			if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != snp_leaf)
-				return; /* GP already done and CBs recorded. */
 			spin_lock_irqsave_rcu_node(snp, flags);
 			snp_seq = snp->srcu_have_cbs[idx];
 			if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
@@ -935,9 +935,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 	if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
 		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 
-	/* If grace period not already done and none in progress, start it. */
-	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
-	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
+	/* If grace period not already in progress, start it. */
+	if (rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
 		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
 		srcu_gp_start(ssp);
 
-- 
2.31.1