The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: 07c54cc5988f19c9642fd463c2dbdac7fc52f777 Gitweb: https://git.kernel.org/tip/07c54cc5988f19c9642fd463c2dbdac7fc52f777 Author: Oleg Nesterov oleg@redhat.com AuthorDate: Tue, 28 May 2024 14:20:19 +02:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Mon, 10 Jun 2024 20:18:13 +02:00

tick/nohz_full: Don't abuse smp_call_function_single() in tick_setup_device()

After the recent commit 5097cbcb38e6 ("sched/isolation: Prevent boot crash when the boot CPU is nohz_full") the kernel no longer crashes, but there is another problem.

In this case tick_setup_device() calls tick_take_do_timer_from_boot() to update tick_do_timer_cpu and this triggers the WARN_ON_ONCE(irqs_disabled) in smp_call_function_single().

Kill tick_take_do_timer_from_boot() and just use WRITE_ONCE(), the new comment explains why this is safe (thanks Thomas!).

Fixes: 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be nohz_full") Signed-off-by: Oleg Nesterov oleg@redhat.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240528122019.GA28794@redhat.com Link: https://lore.kernel.org/all/20240522151742.GA10400@redhat.com --- kernel/time/tick-common.c | 42 ++++++++++++-------------------------- 1 file changed, 14 insertions(+), 28 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c index d88b130..a47bcf7 100644 --- a/kernel/time/tick-common.c +++ b/kernel/time/tick-common.c @@ -178,26 +178,6 @@ void tick_setup_periodic(struct clock_event_device *dev, int broadcast) } }

-#ifdef CONFIG_NO_HZ_FULL -static void giveup_do_timer(void *info) -{ - int cpu = *(unsigned int *)info; - - WARN_ON(tick_do_timer_cpu != smp_processor_id()); - - tick_do_timer_cpu = cpu; -} - -static void tick_take_do_timer_from_boot(void) -{ - int cpu = smp_processor_id(); - int from = tick_do_timer_boot_cpu; - - if (from >= 0 && from != cpu) - smp_call_function_single(from, giveup_do_timer, &cpu, 1); -} -#endif - /* * Setup the tick device */ @@ -221,19 +201,25 @@ static void tick_setup_device(struct tick_device *td, tick_next_period = ktime_get(); #ifdef CONFIG_NO_HZ_FULL /* - * The boot CPU may be nohz_full, in which case set - * tick_do_timer_boot_cpu so the first housekeeping - * secondary that comes up will take do_timer from - * us. + * The boot CPU may be nohz_full, in which case the + * first housekeeping secondary will take do_timer() + * from it. */ if (tick_nohz_full_cpu(cpu)) tick_do_timer_boot_cpu = cpu;

- } else if (tick_do_timer_boot_cpu != -1 && - !tick_nohz_full_cpu(cpu)) { - tick_take_do_timer_from_boot(); + } else if (tick_do_timer_boot_cpu != -1 && !tick_nohz_full_cpu(cpu)) { tick_do_timer_boot_cpu = -1; - WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu); + /* + * The boot CPU will stay in periodic (NOHZ disabled) + * mode until clocksource_done_booting() called after + * smp_init() selects a high resolution clocksource and + * timekeeping_notify() kicks the NOHZ stuff alive. + * + * So this WRITE_ONCE can only race with the READ_ONCE + * check in tick_periodic() but this race is harmless. + */ + WRITE_ONCE(tick_do_timer_cpu, cpu); #endif }