Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

Oleg Nesterov
2024-05-07T14:54:49Z

Hello,

I feel I don't really like this patch but I am travelling without my working
laptop, can't read the source code ;) Quite possibly I am wrong, I'll return
to this when I get back on May 10.

Oleg.

On 05/07, Uladzislau Rezki (Sony) wrote:
>
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> The rcu_sync structure's ->gp_count field is updated under the protection
> of ->rss_lock, but read locklessly, and KCSAN noted the data race.
> This commit therefore uses WRITE_ONCE() to do this update to clearly
> document its racy nature.
>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
> kernel/rcu/sync.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index 86df878a2fee..6c2bd9001adc 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
> * we are called at early boot time but this shouldn't happen.
> */
> }
> - rsp->gp_count++;
> + WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
> spin_unlock_irq(&rsp->rss_lock);
>
> if (gp_state == GP_IDLE) {
> @@ -151,11 +151,15 @@ void rcu_sync_enter(struct rcu_sync *rsp)
> */
> void rcu_sync_exit(struct rcu_sync *rsp)
> {
> + int gpc;
> +
> WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
> WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
>
> spin_lock_irq(&rsp->rss_lock);
> - if (!--rsp->gp_count) {
> + gpc = rsp->gp_count - 1;
> + WRITE_ONCE(rsp->gp_count, gpc);
> + if (!gpc) {
> if (rsp->gp_state == GP_PASSED) {
> WRITE_ONCE(rsp->gp_state, GP_EXIT);
> rcu_sync_call(rsp);
> --
> 2.39.2
>