Re: [PATCH 2/2] clocksource: Defer preempt_disable() after clocksource_verify_choose_cpus()

[Waiman Long <llong@redhat.com>]

On 1/24/25 3:37 PM, Paul E. McKenney wrote:
> On Fri, Jan 24, 2025 at 12:25:19PM -0800, Paul E. McKenney wrote:
>> On Fri, Jan 24, 2025 at 01:59:23PM -0500, Waiman Long wrote:
>>> The following bug report happened in a PREEMPT_RT kernel.
>>>
>>> [   30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
>>> [   30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
>>> [   30.962673] preempt_count: 1, expected: 0
>>> [   30.962676] RCU nest depth: 0, expected: 0
>>> [   30.962680] 3 locks held by kwatchdog/2012:
>>> [   30.962684]  #0: ffffffff8af2da60 (clocksource_mutex){+.+.}-{3:3}, at: clocksource_watchdog_kthread+0x13/0x50
>>> [   30.967703]  #1: ffffffff8aa8d4d0 (cpu_hotplug_lock){++++}-{0:0}, at: clocksource_verify_percpu.part.0+0x5c/0x330
>>> [   30.972774]  #2: ffff9fe02f5f33e0 ((batched_entropy_u32.lock)){+.+.}-{2:2}, at: get_random_u32+0x4f/0x110
>>> [   30.977827] Preemption disabled at:
>>> [   30.977830] [<ffffffff88c1fe56>] clocksource_verify_percpu.part.0+0x66/0x330
>>> [   30.982837] CPU: 33 PID: 2012 Comm: kwatchdog Not tainted 5.14.0-503.23.1.el9_5.x86_64+rt-debug #1
>>> [   30.982843] Hardware name: HPE ProLiant DL385 Gen10 Plus/ProLiant DL385 Gen10 Plus, BIOS A42 04/29/2021
>>> [   30.982846] Call Trace:
>>> [   30.982850]  <TASK>
>>> [   30.983821]  dump_stack_lvl+0x57/0x81
>>> [   30.983821]  __might_resched.cold+0xf4/0x12f
>>> [   30.983824]  rt_spin_lock+0x4c/0x100
>>> [   30.988833]  get_random_u32+0x4f/0x110
>>> [   30.988833]  clocksource_verify_choose_cpus+0xab/0x1a0
>>> [   30.988833]  clocksource_verify_percpu.part.0+0x6b/0x330
>>> [   30.993894]  __clocksource_watchdog_kthread+0x193/0x1a0
>>> [   30.993898]  clocksource_watchdog_kthread+0x18/0x50
>>> [   30.993898]  kthread+0x114/0x140
>>> [   30.993898]  ret_from_fork+0x2c/0x50
>>> [   31.002864]  </TASK>
>>>
>>> It is due to the fact that get_random_u32() is called in
>>> clocksource_verify_choose_cpus() with preemption disabled.
>>> If crng_ready() is true by the time get_random_u32() is called, The
>>> batched_entropy_32 local lock will be acquired. In PREEMPT_RT kernel,
>>> it is a rtmutex and we can't acquire it with preemption disabled.
>>>
>>> To avoid this problem, we can't call get_random_u32() with preemption
>>> disabled. However, smp_processor_id() has to be called with preemption
>>> disabled though and we have to exclude the current CPU from the
>>> cpus_chosen list to be tested.
>>>
>>> Extract current CPU removal code out from
>>> clocksource_verify_choose_cpus() and defer the preempt_disable()
>>> call to after clocksource_verify_choose_cpus() and before
>>> current CPU removal. Also use raw_smp_processor_id() in
>>> clocksource_verify_choose_cpus().
>>>
>>> Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>> Good catch!
>>
>> But we don't need cryptographically secure random numbers (or blistering
>> speed) here.  Substituting something like torture_random()%nr_cpu_ids
>> for get_random_u32_below() work?
> I suppose I should add my concern...  If we don't have preemption disabled
> across this code, we cannot reliably avoid attempting to IPI ourselves.

Does the CPU choosing process itsself needs to have preemption disabled? 
I thought it was because of need to use smp_processor_id() and have the 
current CPU excluded. Preemption is still disabled after that.

Cheers,
Longman