If an exiting non-autoreaping task has already passed exit_notify() and
calls handle_posix_cpu_timers() from IRQ, it can be reaped by its parent
or debugger right after unlock_task_sighand().

If a concurrent posix_cpu_timer_del() runs at that moment, it won't be
able to detect timer->it.cpu.firing != 0: cpu_timer_task_rcu() and/or
lock_task_sighand() will fail.

Add the tsk->exit_state check into run_posix_cpu_timers() to fix this.

This fix is not needed if CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, because
exit_task_work() is called before exit_notify(). But the check still
makes sense, task_work_add(&tsk->posix_cputimers_work.work) will fail
anyway in this case.

Cc: stable@vger.kernel.org
Reported-by: Benoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed4138e ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Move this API to the canonical timer_*() namespace.

[ tglx: Redone against pre rc1 ]

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/aB2X0jCKQO56WdMt@gmail.com

Pull timer core updates from Thomas Gleixner:
 "Updates for the time/timer core code:

   - Rework the initialization of the posix-timer kmem_cache and move
     the cache pointer into the timer_data structure to prevent false
     sharing

   - Switch the alarmtimer code to lock guards

   - Improve the CPU selection criteria in the per CPU validation of the
     clocksource watchdog to avoid arbitrary selections (or omissions)
     on systems with a small number of CPUs

   - The usual cleanups and improvements"

* tag 'timers-core-2025-05-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  tick/nohz: Remove unused tick_nohz_full_add_cpus_to()
  clocksource: Fix the CPUs' choice in the watchdog per CPU verification
  alarmtimer: Switch spin_{lock,unlock}_irqsave() to guards
  alarmtimer: Remove dead return value in clock2alarm()
  time/jiffies: Change register_refined_jiffies() to void __init
  timers: Remove unused __round_jiffies(_up)
  posix-timers: Initialize cache early and move pointer into __timer_data

Pull timer cleanups from Thomas Gleixner:
 "Another set of timer API cleanups:

    - Convert init_timer*(), try_to_del_timer_sync() and
      destroy_timer_on_stack() over to the canonical timer_*()
      namespace convention.

  There is another large conversion pending, which has not been included
  because it would have caused a gazillion of merge conflicts in next.
  The conversion scripts will be run towards the end of the merge window
  and a pull request sent once all conflict dependencies have been
  merged"

* tag 'timers-cleanups-2025-05-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  treewide, timers: Rename destroy_timer_on_stack() as timer_destroy_on_stack()
  treewide, timers: Rename try_to_del_timer_sync() as timer_delete_sync_try()
  timers: Rename init_timers() as timers_init()
  timers: Rename NEXT_TIMER_MAX_DELTA as TIMER_NEXT_MAX_DELTA
  timers: Rename __init_timer_on_stack() as __timer_init_on_stack()
  timers: Rename __init_timer() as __timer_init()
  timers: Rename init_timer_on_stack_key() as timer_init_key_on_stack()
  timers: Rename init_timer_key() as timer_init_key()

Right now, if the clocksource watchdog detects a clocksource skew, it might
perform a per CPU check, for example in the TSC case on x86.  In other
words: supposing TSC is detected as unstable by the clocksource watchdog
running at CPU1, as part of marking TSC unstable the kernel will also run a
check of TSC readings on some CPUs to be sure it is synced between them
all.

But that check happens only on some CPUs, not all of them; this choice is
based on the parameter "verify_n_cpus" and in some random cpumask
calculation. So, the watchdog runs such per CPU checks on up to
"verify_n_cpus" random CPUs among all online CPUs, with the risk of
repeating CPUs (that aren't double checked) in the cpumask random
calculation.

But if "verify_n_cpus" > num_online_cpus(), it should skip the random
calculation and just go ahead and check the clocksource sync between
all online CPUs, without the risk of skipping some CPUs due to
duplicity in the random cpumask calculation.

Tests in a 4 CPU laptop with TSC skew detected led to some cases of the per
CPU verification skipping some CPU even with verify_n_cpus=8, due to the
duplicity on random cpumask generation. Skipping the randomization when the
number of online CPUs is smaller than verify_n_cpus, solves that.

Suggested-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/all/20250323173857.372390-1-gpiccoli@igalia.com

Move this API to the canonical timer_*() namespace.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250507175338.672442-10-mingo@kernel.org

Move this API to the canonical timer_*() namespace.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250507175338.672442-9-mingo@kernel.org

Move this API to the canonical timers_*() namespace.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250507175338.672442-8-mingo@kernel.org

Move this macro to the canonical TIMER_* namespace.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250507175338.672442-7-mingo@kernel.org

Move this API to the canonical timer_*() namespace.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250507175338.672442-4-mingo@kernel.org