sched/mmcid: Prevent live lock on task to CPU mode transition

sched/mmcid: Prevent live lock on task to CPU mode transition

Ihor reported a BPF CI failure which turned out to be a live lock in the
MM_CID management. The scenario is:

A test program creates the 5th thread, which means the MM_CID users become
more than the number of CPUs (four in this example), so it switches to per
CPU ownership mode.

At this point each live task of the program has a CID associated. Assume
thread creation order assignment for simplicity.

   T0     CID0  runs fork() and creates T4
   T1 	  CID1
   T2 	  CID2
   T3 	  CID3
   T4       ---   not visible yet

T0 sets mm_cid::percpu = true and transfers its own CID to CPU0 where it
runs on and then starts the fixup which walks through the threads to
transfer the per task CIDs either to the CPU the task is running on or drop
it back into the pool if the task is not on a CPU.

During that T1 - T3 are free to schedule in and out before the fixup caught
up with them. Going through all possible permutations with a python script
revealed a few problematic cases. The most trivial one is:

   T1 schedules in on CPU1 and observes percpu == true, so it transfers
      its CID to CPU1

   T1 is migrated to CPU2 and schedule in observes percpu == true, but
      CPU2 does not have a CID associated and T1 transferred its own to
      CPU1

      So it has to allocate one with CPU2 runqueue lock held, but the
      pool is empty, so it keeps looping in mm_get_cid().

Now T0 reaches T1 in the thread walk and tries to lock the corresponding
runqueue lock, which is held causing a full live lock.

There is a similar scenario in the reverse direction of switching from per
CPU to task mode which is way more obvious and got therefore addressed by
an intermediate mode. In this mode the CIDs are marked with MM_CID_TRANSIT,
which means that they are neither owned by the CPU nor by the task. When a
task schedules out with a transit CID it drops the CID back into the pool
making it available for others to use temporarily. Once the task which
initiated the mode switch finished the fixup it clears the transit mode and
the process goes back into per task ownership mode.

Unfortunately this insight was not mapped back to the task to CPU mode
switch as the above described scenario was not considered in the analysis.

Apply the same transit mechanism to the task to CPU mode switch to handle
these problematic cases correctly.

As with the CPU to task transition this results in a potential temporary
contention on the CID bitmap, but that's only for the time it takes to
complete the transition. After that it stays in steady mode which does not
touch the bitmap at all.

Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Closes: https://lore.kernel.org/2b7463d7-0f58-4e34-9775-6e2115cfb971@linux.dev
Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20260201192834.897115238@kernel.org