On 25 Sep 2021 at 03:21, Eric Sandeen wrote: > On 9/24/21 9:09 AM, Chandan Babu R wrote: >> From: Dave Chinner <dchinner@xxxxxxxxxx> >> The upcoming buffer cache rework/kerenl sync-up requires atomic >> variables. I could use C++11 atomics build into GCC, but they are a >> pain to work with and shoe-horn into the kernel atomic variable API. >> Much easier is to introduce a dependency on liburcu - the userspace >> RCU library. This provides atomic variables that very closely match >> the kernel atomic variable API, and it provides a very similar >> memory model and memory barrier support to the kernel. And we get >> RCU support that has an identical interface to the kernel and works >> the same way. >> Hence kernel code written with RCU algorithms and atomic variables >> will just slot straight into the userspace xfsprogs code without us >> having to think about whether the lockless algorithms will work in >> userspace or not. This reduces glue and hoop jumping, and gets us >> a step closer to having the entire userspace libxfs code MT safe. >> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx> >> [chandan.babu@xxxxxxxxxx: Add m4 macros to detect availability of liburcu] > > Thanks for fixing that up. I had tried to use rcu_init like Dave originally > had, and I like that better in general, but I had trouble with it - I guess > maybe it gets redefined based on memory model magic and the actual symbol > "rcu_init" maybe isn't available? I didn't dig very much. Yes, rcu_init() gets defined to one of the variants of userspace rcu. Hence there is no function named rcu_init(). > > Also, dumb question from me - how do we know where we need the > rcu_[un]register_thread() calls? Will it be obvious if we miss it > in the future? "each thread must invoke this function before its > first call to rcu_read_lock() or call_rcu()." Unfortunately, I don't think there is an obvious method to detect if calls to rcu_[un]register_thread() is missing from any code changes implemented in the future. PS: I am no expert at RCU. My knowledge about RCU is limited to what I could learn by reading articles on the world wide web. -- chandan
