[PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

[Joel Fernandes]

> On Jul 12, 2023, at 10:02 PM, Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
> 
> 
> 
>> On 2023/7/13 08:32, Joel Fernandes wrote:
>> On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote:
>> [..]
>>>> As such this patch looks correct to me, one thing I noticed is that
>>>> you can check rcu_is_watching() like the lockdep-enabled code does.
>>>> That will tell you also if a reader-section is possible because in
>>>> extended-quiescent-states, RCU readers should be non-existent or
>>>> that's a bug.
>>>> 
>>> Please correct me if I am wrong, reading from the comment in
>>> kernel/rcu/update.c rcu_read_lock_held_common()
>>> ..
>>>   * The reason for this is that RCU ignores CPUs that are
>>>  * in such a section, considering these as in extended quiescent state,
>>>  * so such a CPU is effectively never in an RCU read-side critical section
>>>  * regardless of what RCU primitives it invokes.
>>> 
>>> It seems rcu will treat this as lock not held rather than a fact that
>>> lock is not held. Is my understanding correct?
>> If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you
>> mean it is not a fact for erofs?
> 
> I'm not sure if I get what you mean, EROFS doesn't take any RCU read lock

We are discussing the case 3 you mentioned below.

> here:
> 
> z_erofs_decompressqueue_endio() is actually a "bio->bi_end_io", previously
> which can be called under two scenarios:
> 
> 1) under softirq context, which is actually part of device I/O compleltion;
> 
> 2) under threaded context, like what dm-verity or likewise calls.
> 
> But EROFS needs to decompress in a threaded context anyway, so we trigger
> a workqueue to resolve the case 1).
> 
> 
> Recently, someone reported there could be some case 3) [I think it was
> introduced recently but I have no time to dig into it]:
> 
> case 3: under RCU read lock context, which is shown by this:
> https://lore.kernel.org/r/4a8254eb-ac39-1e19-3d82-417d3a7b9f94@linux.alibaba.com/T/#u
> 
> and such RCU read lock is taken in __blk_mq_run_dispatch_ops().
> 
> But as the commit shown, we only need to trigger a workqueue for case 1)
> and 3) due to performance reasons.
> 
> Hopefully I show it more clear.

Makes sense. Thanks,

 - Joel

> 
> Thanks,
> Gao Xiang