On Wed, 24 Jun 2020 at 11:01, Peter Zijlstra [off-list ref] wrote:

On Tue, Jun 23, 2020 at 10:24:04PM +0200, Peter Zijlstra wrote:

quoted

On Tue, Jun 23, 2020 at 08:12:32PM +0200, Peter Zijlstra wrote:

quoted

Fair enough; I'll rip it all up and boot a KCSAN kernel, see what if
anything happens.

OK, so the below patch doesn't seem to have any nasty recursion issues
here. The only 'problem' is that lockdep now sees report_lock can cause
deadlocks.

It is completely right about it too, but I don't suspect there's much we
can do about it, it's pretty much the standard printk() with scheduler
locks held report.

So I've been getting tons and tons of this:

[   60.471348] ==================================================================
[   60.479427] BUG: KCSAN: data-race in __rcu_read_lock / __rcu_read_unlock
[   60.486909]
[   60.488572] write (marked) to 0xffff88840fff1cf0 of 4 bytes by interrupt on cpu 1:
[   60.497026]  __rcu_read_lock+0x37/0x60
[   60.501214]  cpuacct_account_field+0x1b/0x170
[   60.506081]  task_group_account_field+0x32/0x160
[   60.511238]  account_system_time+0xe6/0x110
[   60.515912]  update_process_times+0x1d/0xd0
[   60.520585]  tick_sched_timer+0xfc/0x180
[   60.524967]  __hrtimer_run_queues+0x271/0x440
[   60.529832]  hrtimer_interrupt+0x222/0x670
[   60.534409]  __sysvec_apic_timer_interrupt+0xb3/0x1a0
[   60.540052]  asm_call_on_stack+0x12/0x20
[   60.544434]  sysvec_apic_timer_interrupt+0xba/0x130
[   60.549882]  asm_sysvec_apic_timer_interrupt+0x12/0x20
[   60.555621]  delay_tsc+0x7d/0xe0
[   60.559226]  kcsan_setup_watchpoint+0x292/0x4e0
[   60.564284]  __rcu_read_unlock+0x73/0x2c0
[   60.568763]  __unlock_page_memcg+0xda/0xf0
[   60.573338]  unlock_page_memcg+0x32/0x40
[   60.577721]  page_remove_rmap+0x5c/0x200
[   60.582104]  unmap_page_range+0x83c/0xc10
[   60.586582]  unmap_single_vma+0xb0/0x150
[   60.590963]  unmap_vmas+0x81/0xe0
[   60.594663]  exit_mmap+0x135/0x2b0
[   60.598464]  __mmput+0x21/0x150
[   60.601970]  mmput+0x2a/0x30
[   60.605176]  exit_mm+0x2fc/0x350
[   60.608780]  do_exit+0x372/0xff0
[   60.612385]  do_group_exit+0x139/0x140
[   60.616571]  __do_sys_exit_group+0xb/0x10
[   60.621048]  __se_sys_exit_group+0xa/0x10
[   60.625524]  __x64_sys_exit_group+0x1b/0x20
[   60.630189]  do_syscall_64+0x6c/0xe0
[   60.634182]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   60.639820]
[   60.641485] read to 0xffff88840fff1cf0 of 4 bytes by task 2430 on cpu 1:
[   60.648969]  __rcu_read_unlock+0x73/0x2c0
[   60.653446]  __unlock_page_memcg+0xda/0xf0
[   60.658019]  unlock_page_memcg+0x32/0x40
[   60.662400]  page_remove_rmap+0x5c/0x200
[   60.666782]  unmap_page_range+0x83c/0xc10
[   60.671259]  unmap_single_vma+0xb0/0x150
[   60.675641]  unmap_vmas+0x81/0xe0
[   60.679341]  exit_mmap+0x135/0x2b0
[   60.683141]  __mmput+0x21/0x150
[   60.686647]  mmput+0x2a/0x30
[   60.689853]  exit_mm+0x2fc/0x350
[   60.693458]  do_exit+0x372/0xff0
[   60.697062]  do_group_exit+0x139/0x140
[   60.701248]  __do_sys_exit_group+0xb/0x10
[   60.705724]  __se_sys_exit_group+0xa/0x10
[   60.710201]  __x64_sys_exit_group+0x1b/0x20
[   60.714872]  do_syscall_64+0x6c/0xe0
[   60.718864]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   60.724503]
[   60.726156] Reported by Kernel Concurrency Sanitizer on:
[   60.732089] CPU: 1 PID: 2430 Comm: sshd Not tainted 5.8.0-rc2-00186-gb4ee11fe08b3-dirty #303
[   60.741510] Hardware name: Intel Corporation S2600GZ/S2600GZ, BIOS SE5C600.86B.02.02.0002.122320131210 12/23/2013
[   60.752957] ==================================================================

And I figured a quick way to get rid of that would be something like the
below, seeing how volatile gets auto annotated... but that doesn't seem
to actually work.

What am I missing?

There's one more in include/linux/rcupdate.h. I suggested this at some point:

    https://lore.kernel.org/lkml/20200220213317.GA35033@google.com/ (local)

To avoid volatiles as I don't think they are needed here.

[ Still testing your other patches for KCSAN, will send another reply there. ]

Thanks,
-- Marco