When a group event is updated with its expiry unchanged but a different
CPU, that target change may go unnoticed and the event may be propagated
up with a stale CPU value. The following depicts a scenario that has
been actually observed:

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = TGRP1:0 (T0)
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T0
      /         \
    0 (T0)       1 (T1)
    idle         idle

0) The hierarchy has 3 levels. The left part (GRP1:0) is all idle,
including CPU 0 and CPU 1 which have a timer each: T0 and T1. They have
the same expiry value.

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = KTIME_MAX
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T0
      /         \
    0 (T0)       1 (T1)
    idle         idle

1) The migrator in GRP1:1 handles remotely T0. The event is dequeued
from the top and T0 executed.

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = KTIME_MAX
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T1
      /         \
    0            1 (T1)
    idle         idle

2) The migrator in GRP1:1 fetches the next timer for CPU 0 and finds
none. But it updates the events from its groups, starting with GRP0:0
which now has T1 as its next event. So far so good.

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = KTIME_MAX
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T1
      /         \
    0            1 (T1)
    idle         idle

3) The migrator in GRP1:1 proceeds upward and updates the events in
GRP1:0. The child event TGRP0:0 is found queued with the same expiry
as before. And therefore it is left unchanged. However the target CPU
is not the same but that fact is ignored so TGRP0:0 still points to
CPU 0 when it should point to CPU 1.

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = TGRP1:0 (T0)
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T1
      /         \
    0            1 (T1)
    idle         idle

4) The propagation has reached the top level and TGRP1:0, having TGRP0:0
as its first event, also wrongly points to CPU 0. TGRP1:0 is added to
the top level group.

                       [GRP2:0]
                   migrator = GRP1:1
                   active   = GRP1:1
                   nextevt  = KTIME_MAX
                    /              \
               [GRP1:0]           [GRP1:1]
            migrator = NONE       [...]
            active   = NONE
            nextevt  = TGRP0:0 (T0)
            /           \
        [GRP0:0]       [...]
      migrator = NONE
      active   = NONE
      nextevt  = T1
      /         \
    0            1 (T1)
    idle         idle

5) The migrator in GRP1:1 dequeues the next event in top level pointing
to CPU 0. But since it actually doesn't see any real event in CPU 0, it
early returns.

6) T1 is left unhandled until either CPU 0 or CPU 1 wake up.

Some other bad scenario may involve trees with just two levels.

Fix this with unconditionally updating the CPU of the child event before
considering to early return while updating a queued event with an
unchanged expiry value.

Fixes: 7ee98877 ("timers: Implement the hierarchical pull model")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Link: https://lore.kernel.org/r/Zg2Ct6M2RJAYHgCB@localhost.localdomain