[PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC
Gao Xiang
hsiangkao at linux.alibaba.com
Thu Jul 13 20:16:52 PDT 2023
On 2023/7/14 10:16, Paul E. McKenney wrote:
> On Thu, Jul 13, 2023 at 09:33:35AM -0700, Paul E. McKenney wrote:
>> On Thu, Jul 13, 2023 at 11:33:24AM -0400, Joel Fernandes wrote:
...
>>>
>>> >From what Sandeep described, the code path is in an RCU reader. My
>>> question is more, why doesn't it use SRCU instead since it clearly
>>> does so if BLK_MQ_F_BLOCKING. What are the tradeoffs? IMHO, a deeper
>>> dive needs to be made into that before concluding that the fix is to
>>> use rcu_read_lock_any_held().
>>
>> How can this be solved?
>>
>> 1. Always use a workqueue. Simple, but is said to have performance
>> issues.
>>
>> 2. Pass a flag in that indicates whether or not the caller is in an
>> RCU read-side critical section. Conceptually simple, but might
>> or might not be reasonable to actually implement in the code as
>> it exists now. (You tell me!)
>>
>> 3. Create a function in z_erofs that gives you a decent
>> approximation, maybe something like the following.
>>
>> 4. Other ideas here.
>
> 5. #3 plus make the corresponding Kconfig option select
> PREEMPT_COUNT, assuming that any users needing compression in
> non-preemptible kernels are OK with PREEMPT_COUNT being set.
> (Some users of non-preemptible kernels object strenuously
> to the added overhead from CONFIG_PREEMPT_COUNT=y.)
I'm not sure if it's a good idea, we need to work on
CONFIG_PREEMPT_COUNT=n (why not?), we could just always trigger a
workqueue for this.
Anyway, before we proceed, I also think it'd be better to get some
performance numbers first for this (e.g. with dm-verity) and record
the numbers in the commit message to justify this. Otherwise, I guess
the same question will be raised again and again.
Thanks,
Gao Xiang
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