On 4/30/2025 12:14 PM, Joel Fernandes wrote:
>
>
> On 4/30/2025 10:57 AM, Z qiang wrote:
>>>
>>>
>>>
>>> On 4/28/2025 6:59 AM, Z qiang wrote:
>>>>>
>>>>> Le Mon, Apr 28, 2025 at 05:54:03PM +0800, Zqiang a écrit :
>>>>>> For Preempt-RT kernel, when enable CONFIG_PROVE_RCU Kconfig,
>>>>>> disable local bh in rcuc kthreads will not affect preempt_count(),
>>>>>> this resulted in the following splat:
>>>>>>
>>>>>> WARNING: suspicious RCU usage
>>>>>> kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state!
>>>>>> stack backtrace:
>>>>>> CPU: 0 UID: 0 PID: 22 Comm: rcuc/0
>>>>>> Call Trace:
>>>>>> [ 0.407907] <TASK>
>>>>>> [ 0.407910] dump_stack_lvl+0xbb/0xd0
>>>>>> [ 0.407917] dump_stack+0x14/0x20
>>>>>> [ 0.407920] lockdep_rcu_suspicious+0x133/0x210
>>>>>> [ 0.407932] rcu_rdp_is_offloaded+0x1c3/0x270
>>>>>> [ 0.407939] rcu_core+0x471/0x900
>>>>>> [ 0.407942] ? lockdep_hardirqs_on+0xd5/0x160
>>>>>> [ 0.407954] rcu_cpu_kthread+0x25f/0x870
>>>>>> [ 0.407959] ? __pfx_rcu_cpu_kthread+0x10/0x10
>>>>>> [ 0.407966] smpboot_thread_fn+0x34c/0xa50
>>>>>> [ 0.407970] ? trace_preempt_on+0x54/0x120
>>>>>> [ 0.407977] ? __pfx_smpboot_thread_fn+0x10/0x10
>>>>>> [ 0.407982] kthread+0x40e/0x840
>>>>>> [ 0.407990] ? __pfx_kthread+0x10/0x10
>>>>>> [ 0.407994] ? rt_spin_unlock+0x4e/0xb0
>>>>>> [ 0.407997] ? rt_spin_unlock+0x4e/0xb0
>>>>>> [ 0.408000] ? __pfx_kthread+0x10/0x10
>>>>>> [ 0.408006] ? __pfx_kthread+0x10/0x10
>>>>>> [ 0.408011] ret_from_fork+0x40/0x70
>>>>>> [ 0.408013] ? __pfx_kthread+0x10/0x10
>>>>>> [ 0.408018] ret_from_fork_asm+0x1a/0x30
>>>>>> [ 0.408042] </TASK>
>>>>>>
>>>>>> Currently, triggering an rdp offloaded state change need the
>>>>>> corresponding rdp's CPU goes offline, and at this time the rcuc
>>>>>> kthreads has already in parking state. this means the corresponding
>>>>>> rcuc kthreads can safely read offloaded state of rdp while it's
>>>>>> corresponding cpu is online.
>>>>>>
>>>>>> This commit therefore add rdp->rcu_cpu_kthread_task check for
>>>>>> Preempt-RT kernels.
>>>>>>
>>>>>> Signed-off-by: Zqiang <qiang.zhang1...@gmail.com>
>>>>>> ---
>>>>>> kernel/rcu/tree_plugin.h | 4 +++-
>>>>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>>>>>> index 003e549f6514..fe728eded36e 100644
>>>>>> --- a/kernel/rcu/tree_plugin.h
>>>>>> +++ b/kernel/rcu/tree_plugin.h
>>>>>> @@ -31,7 +31,9 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
>>>>>> lockdep_is_held(&rcu_state.nocb_mutex) ||
>>>>>> (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
>>>>>> rdp == this_cpu_ptr(&rcu_data)) ||
>>>>>> - rcu_current_is_nocb_kthread(rdp)),
>>>>>> + rcu_current_is_nocb_kthread(rdp) ||
>>>>>> + (IS_ENABLED(CONFIG_PREEMPT_RT) &&
>>>>>> + current == rdp->rcu_cpu_kthread_task)),
>>>>>
>>>>> Isn't it safe also on !CONFIG_PREEMPT_RT ?
>>>>
>>>> For !CONFIG_PREEMPT_RT and in rcuc kthreads, it's also safe,
>>>> but the following check will passed :
>>>>
>>>> (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
>>>> rdp == this_cpu_ptr(&rcu_data))
>>>
>>> I think the fact that it already passes for !PREEMPT_RT does not matter,
>>> because
>>> it simplifies the code so drop the PREEMPT_RT check?
>>>
>>> Or will softirq_count() not work? It appears to have special casing for
>>> PREEMPT_RT's local_bh_disable():
>>>
>>> ( ( !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) ||
>>> softirq_count() )
>>> && rdp == this_cpu_ptr(&rcu_data)) )
>>
>> Thank you for Joel's reply, I also willing to accept such
>> modifications and resend :) .
> Thanks, I am Ok with either approach whichever you and Frederic together
> decide.
> I can then pull this in for the v6.16 merge window once you resend, thanks!
>
Frederic, there are a couple of ways we can move forward hear. Does the
softirq_count() approach sound good to you? If yes, I can fixup the patch
myself.
I am also Ok at this point to take it in for 6.16, though I've also stored it in
my rcu/dev branch for Neeraj's 6.17 PR, just in case :)
- Joel
