On 2023/7/22 6:37, Paul E. McKenney wrote:
> On Fri, Jul 21, 2023 at 03:57:15PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
>> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>>
>> j = jiffies;
>> js = READ_ONCE(rcu_state.jiffies_stall); (1)
>> if (ULONG_CMP_LT(j, js)) (2)
>> return;
>>
>> if (cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) (3)
>> didstall = true;
>>
>> if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) { (4)
>> jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>> WRITE_ONCE(rcu_state.jiffies_stall, jn);
>> }
>>
>> For ease of description, the pseudo code is extracted as above. First,
>> assume that only one CPU is operating, the condition 'didstall' is true
>> means that (3) succeeds. That is, the value of rcu_state.jiffies_stall
>> must be 'jn'.
>>
>> Then, assuming that another CPU is also operating at the same time, there
>> are two cases:
>> 1. That CPU sees the updated result at (1), it returns at (2).
>> 2. That CPU does not see the updated result at (1), it fails at (3) and
>> cmpxchg returns jn. So that, didstall cannot be true.
>
> The history behind this one is that there are races in which the stall
> can end in the midst of check_cpu_stall(). For example, when the activity
> of producing the stall warning breaks things loose.
>
> And yes, long ago, I figured that if things had been static for so
> many seconds, they were unlikely to change, and thus omitted any and
> all synchronization. The Linux kernel taught me better. ;-)
Okay, got it, thank you
>
> Thanx, Paul
>
>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>> ---
>> kernel/rcu/tree_stall.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index cc884cd49e026a3..371713f3f7d15d9 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -794,7 +794,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>> rcu_ftrace_dump(DUMP_ALL);
>> didstall = true;
>> }
>> - if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
>> + if (didstall) {
>> jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>> WRITE_ONCE(rcu_state.jiffies_stall, jn);
>> }
>> --
>> 2.25.1
>>
> .
>
--
Regards,
Zhen Lei
