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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

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
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.

Thanks,
Gao Xiang