On Mon, Jan 23, 2023 at 08:44:25AM +0800, Zhouyi Zhou wrote: > On Mon, Jan 9, 2023 at 11:59 PM Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> wrote: > > > > On Mon, Jan 9, 2023 at 11:25 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > On Mon, Jan 09, 2023 at 01:51:29PM +0100, Frederic Weisbecker wrote: > > > > 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@xxxxxxxxx> > > > > > --- > > > > > 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@xxxxxx/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. > > > > > > Please also an analysis of why the compiler cannot do any destructive > > > optimizations in this case. Maybe also comments. > > I want to try the analysis above, as a newbie I have taught myself > > LLVM for 3 years in my spare time ;-) > > I am continuing to study LLVM/Clang's optimization behaviour on > tick_do_timer_cpu, it is very interesting and fascinating, but as a > newbie, I still need 3-4 week to go, could you wait for me a little > while? ;-) Please do take the time to get it right and to get it correctly documented. ;-) Thanx, Paul > Many Thanks > Zhouyi > > > > > > > One more thing on my TODO list, but feel free to beat me at it :-) > > Please take your time ;-) Please don't look my next possible email > > as a reminder ;-) > > > > > > I know that feeling! ;-) > > > > > Thanx, Zhouyi > > > Thanx, Paul
