On Fri, Dec 23, 2022 at 10:15:07PM +0100, Frederic Weisbecker wrote:
> RCU sometimes needs to perform a delayed wake up for specific kthreads
> handling offloaded callbacks (RCU_NOCB). These wakeups are performed
> by timers and upon entry to idle (also to guest and to user on nohz_full).
>
> However the delayed wake-up on kernel exit is actually performed after
> the thread flags are fetched towards the fast path check for work to
> do on exit to user. As a result, and if there is no other pending work
> to do upon that kernel exit, the current task will resume to userspace
> with TIF_RESCHED set and the pending wake up ignored.
>
> Fix this with fetching the thread flags _after_ the delayed RCU-nocb
> kthread wake-up.
>
> Fixes: 47b8ff194c1f ("entry: Explicitly flush pending rcuog wakeup before last rescheduling point")
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Queued and pushed, thank you!
Thanx, Paul
> ---
> kernel/entry/common.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 846add8394c4..a134e26b58c6 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -192,13 +192,14 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>
> static void exit_to_user_mode_prepare(struct pt_regs *regs)
> {
> - unsigned long ti_work = read_thread_flags();
> + unsigned long ti_work;
>
> lockdep_assert_irqs_disabled();
>
> /* Flush pending rcuog wakeup before the last need_resched() check */
> tick_nohz_user_enter_prepare();
>
> + ti_work = read_thread_flags();
> if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
> ti_work = exit_to_user_mode_loop(regs, ti_work);
>
> --
> 2.25.1
>
