Messages in this thread | | | Date | Fri, 14 Jul 2023 11:16:52 +0800 | Subject | Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC | From | Gao Xiang <> |
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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:
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>>> >>> >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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