linux-imx

LF-226 cpufreq: Avoid leaving stale IRQ work items during CPU offline

The scheduler code calling cpufreq_update_util() may run during CPU
offline on the target CPU after the IRQ work lists have been flushed
for it, so the target CPU should be prevented from running code that
may queue up an IRQ work item on it at that point.

Unfortunately, that may not be the case if dvfs_possible_from_any_cpu
is set for at least one cpufreq policy in the system, because that
allows the CPU going offline to run the utilization update callback
of the cpufreq governor on behalf of another (online) CPU in some cases.

If that happens, the cpufreq governor callback may queue up an IRQ work
on the CPU running it, which is going offline, and the IRQ work will
not be flushed after that point.  Moreover, that IRQ work cannot be
flushed until the "offlining" CPU goes back online, so if any other
CPU calls irq_work_sync() to wait for the completion of that IRQ work,
it will have to wait until the "offlining" CPU is back online and that
may not happen forever. In particular, a system-wide deadlock may
occur during CPU online as a result of that.

The failing scenario is as follows. CPU0 is the boot CPU, so it creates
a cpufreq policy and becomes the "leader" of it (policy->cpu).  It
cannot go offline, because it is the boot CPU.
Next, other CPUs join the cpufreq policy as they go online and they
leave it when they go offline.  The last CPU to go offline, say CPU3,
may queue up an IRQ work while running the governor callback on behalf
of CPU0 after leaving the cpufreq policy because of the
dvfs_possible_from_any_cpu effect described above.  Then, CPU0 is the
only online CPU in the system and the stale IRQ work is still queued on
CPU3.  When, say, CPU1 goes back online, it will run
irq_work_sync() to wait for that IRQ work to complete and so it will
wait for CPU3 to go back online (which may never happen even in
principle), but (worse yet) CPU0 is waiting for CPU1 at that point too
and a system-wide deadlock occurs.

To address this problem notice that CPUs which cannot run cpufreq
utilization update code for themselves (for example, because they have
left the cpufreq policies that they belonged to), should also be
prevented from running that code on behalf of the other CPUs that
belong to a cpufreq policy with dvfs_possible_from_any_cpu set and so
in that case the cpufreq_update_util_data pointer of the CPU running
the code must not be NULL as well as for the CPU which is the target
of the cpufreq utilization update in progress.

Accordingly, change cpufreq_this_cpu_can_update() into a regular
function in kernel/sched/cpufreq.c (instead of a static inline in a
header file) and make it check the cpufreq_update_util_data pointer
of the local CPU if dvfs_possible_from_any_cpu is set for the target
cpufreq policy.

Also update the schedutil governor to do the
cpufreq_this_cpu_can_update() check in the non-fast-switch case too
to avoid the stale IRQ work issues.

Fixes: 99d14d0e16fa ("cpufreq: Process remote callbacks from any CPU if the platform permits")
Reported-by: Anson Huang <anson.huang@nxp.com>
Cc: 4.14+ <stable@vger.kernel.org> # 4.14+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>