Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

[Uros Bizjak]

On Wed, Mar 1, 2023 at 5:38 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 1 Mar 2023 11:28:47 +0100
> Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> > These benefits are the reason the change to try_cmpxchg was accepted
> > also in the linear code elsewhere in the linux kernel, e.g. [2,3] to
> > name a few commits, with a thumbs-up and a claim that the new code is
> > actually *clearer* at the merge commit [4].
>
> I'll say it here too. I really like Joel's suggestion of having a
> cmpxchg_success() that does not have the added side effect of modifying the
> old variable.
>
> I think that would allow for the arch optimizations that you are trying to
> achieve, as well as remove the side effect that might cause issues down the
> road.

Attached patch implements this suggestion.

Uros.
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b10b8349bb2a..229263ebba3b 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -709,6 +709,12 @@ static void print_cpu_stall(unsigned long gps)
 	set_preempt_need_resched();
 }
 
+#define cmpxchg_success(ptr, old, new)				\
+({								\
+	__typeof__(*(ptr)) __tmp = (old);			\
+	try_cmpxchg((ptr), &__tmp, (new));			\
+})
+
 static void check_cpu_stall(struct rcu_data *rdp)
 {
 	bool didstall = false;
@@ -760,7 +766,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 	jn = jiffies + ULONG_MAX / 2;
 	if (rcu_gp_in_progress() &&
 	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
-	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+	    cmpxchg_success(&rcu_state.jiffies_stall, js, jn)) {
 
 		/*
 		 * If a virtual machine is stopped by the host it can look to
@@ -778,7 +784,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 
 	} else if (rcu_gp_in_progress() &&
 		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
-		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+		   cmpxchg_success(&rcu_state.jiffies_stall, js, jn)) {
 
 		/*
 		 * If a virtual machine is stopped by the host it can look to