Re: [PATCH linux-next] mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote:
> mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug
> reported by KCSAN.
> 
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
> ---
> During the rcutorture test on linux-next,
> ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan  --kcsan-kmake-arg "CC=clang-12"
> following KCSAN BUG is reported:
> [   35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M
> [   35.400593] ^M
> [   35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M
> [   35.405325]  tick_nohz_idle_stop_tick+0x14c/0x3e0^M
> [   35.407162]  do_idle+0xf3/0x2a0^M
> [   35.408016]  cpu_startup_entry+0x15/0x20^M
> [   35.409084]  start_secondary+0x8f/0x90^M
> [   35.410207]  secondary_startup_64_no_verify+0xe1/0xeb^M
> [   35.411607] ^M
> [   35.412042] no locks held by swapper/3/0.^M
> [   35.413172] irq event stamp: 53048^M
> [   35.414175] hardirqs last  enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M
> [   35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M
> [   35.418988] softirqs last  enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [   35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [   35.423685] ^M
> [   35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M
> [   35.425870]  tick_nohz_next_event+0x233/0x2b0^M
> [   35.427119]  tick_nohz_idle_stop_tick+0x8f/0x3e0^M
> [   35.428386]  do_idle+0xf3/0x2a0^M
> [   35.429265]  cpu_startup_entry+0x15/0x20^M
> [   35.430429]  rest_init+0x20c/0x210^M
> [   35.431382]  arch_call_rest_init+0xe/0x10^M
> [   35.432508]  start_kernel+0x544/0x600^M
> [   35.433519]  secondary_startup_64_no_verify+0xe1/0xeb^M
> 
> fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE

This has been discussed before with passion:

http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/

To me data_race() would be more appropriate but that would need a changelog with
proper analysis of the tick_do_timer_cpu state machine.

One more thing on my TODO list, but feel free to beat me at it :-)

Thanks.