On 25 Sep 2021 at 03:21, Eric Sandeen wrote:

On 9/24/21 9:09 AM, Chandan Babu R wrote:

quoted

From: Dave Chinner <redacted>
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 <redacted>
[chandan.babu@oracle.com: 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