signal: Queue ignored posixtimers on ignore list

Queue posixtimers which have their signal ignored on the ignored list:

   1) When the timer fires and the signal has SIG_IGN set

   2) When SIG_IGN is installed via sigaction() and a timer signal
      is already queued

This only happens when the signal is for a valid timer, which delivered the
signal in periodic mode. One-shot timer signals are correctly dropped.

Due to the lock order constraints (sighand::siglock nests inside
timer::lock) the signal code cannot access any of the timer fields which
are relevant to make this decision, e.g. timer::it_status.

This is addressed by establishing a protection scheme which requires to
lock both locks on the timer side for modifying decision fields in the
timer struct and therefore makes it possible for the signal delivery to
evaluate with only sighand:siglock being held:

  1) Move the NULLification of timer->it_signal into the sighand::siglock
     protected section of timer_delete() and check timer::it_signal in the
     code path which determines whether the signal is dropped or queued on
     the ignore list.

     This ensures that a deleted timer cannot be moved onto the ignore
     list, which would prevent it from being freed on exit() as it is not
     longer in the process' posix timer list.

     If the timer got moved to the ignored list before deletion then it is
     removed from the ignored list under sighand lock in timer_delete().

  2) Provide a new timer::it_sig_periodic flag, which gets set in the
     signal queue path with both timer and sighand locks held if the timer
     is actually in periodic mode at expiry time.

     The ignore list code checks this flag under sighand::siglock and drops
     the signal when it is not set.

     If it is set, then the signal is moved to the ignored list independent
     of the actual state of the timer.

     When the signal is un-ignored later then the signal is moved back to
     the signal queue. On signal delivery the posix timer side decides
     about dropping the signal if the timer was re-armed, dis-armed or
     deleted based on the signal sequence counter check.

     If the thread/process exits then not yet delivered signals are
     discarded which means the reference of the timer containing the
     sigqueue is dropped and frees the timer.

     This is way cheaper than requiring all code paths to lock
     sighand::siglock of the target thread/process on any modification of
     timer::it_status or going all the way and removing pending signals
     from the signal queues on every rearm, disarm or delete operation.

So the protection scheme here is that on the timer side both timer::lock
and sighand::siglock have to be held for modifying

   timer::it_signal
   timer::it_sig_periodic

which means that on the signal side holding sighand::siglock is enough to
evaluate these fields.
                                                                                                                                                                                                                                                                                                                             
In posixtimer_deliver_signal() holding timer::lock is sufficient to do the
sequence validation against timer::it_signal_seq because a concurrent
expiry is waiting on timer::lock to be released.

This completes the SIG_IGN handling and such timers are not longer self
rearmed which avoids pointless wakeups.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20241105064214.120756416@linutronix.de

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