On Wed, Mar 01, 2023 at 07:43:34PM +0100, Uros Bizjak wrote:
> 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.
Please help me out here.
Why on earth are we even discussing making this change to code that
normally never executes? Performance is not a consideration here.
What am I missing here? Is there some sort of forward-progress
issue that this change addresses?
Thanx, Paul
> 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
