timers/migration: Annotate accesses to ignore flag
The group's ignore flag is:
_ read under the group's lock (idle entry, remote expiry)
_ turned on/off under the group's lock (idle entry, remote expiry)
_ turned on locklessly on idle exit
When idle exit or remote expiry clear the "ignore" flag of a group, the
operation must be synchronized against other concurrent idle exit or
remote expiry to make sure the related group timer is never be missed.
To enforce this synchronization, both "ignore" clearing and read are
performed under the group lock.
On the contrary, whether idle entry and remote expiry observe the ignore
flag turned on by a CPU exiting idle is a matter of optimization. If
that flag set is missed or cleared concurrently, the worst outcome is a
migrator wasting time remotely handling a "ghost" timer. This is why the
ignore flag can be set locklessly.
Unfortunately, the related lockless acceses are bare and miss
appropriate annotations. KCSAN rightfully complains:
BUG: KCSAN: data-race in __tmigr_cpu_activate / print_report
write to 0xffff88842fc28004 of 1 bytes by task 0 on cpu 0:
__tmigr_cpu_activate
tmigr_cpu_activate
timer_clear_idle
tick_nohz_restart_sched_tick
tick_nohz_idle_exit
do_idle
cpu_startup_entry
kernel_init
do_initcalls
clear_bss
reserve_bios_regions
common_startup_64
read to 0xffff88842fc28004 of 1 bytes by task 0 on cpu 1:
print_report
kcsan_report_known_origin
kcsan_setup_watchpoint
tmigr_next_groupevt
tmigr_update_events
tmigr_inactive_up
__walk_groups+0x50/0x77
walk_groups
__tmigr_cpu_deactivate
tmigr_cpu_deactivate
__get_next_timer_interrupt
timer_base_try_to_set_idle
tick_nohz_stop_tick
tick_nohz_idle_stop_tick
cpuidle_idle_call
do_idle
Although the relevant accesses could be marked as data_race(), the
"ignore" flag being read several times within the same
tmigr_update_events() function is confusing and error prone. Prefer
reading it once in that function and make use of similar accesses
elsewhere with appropriate comments when necessary.
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202501031612.62e0c498-lkp@intel.com
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
1 file changed
