On Fri, Sep 24, 2021 at 04:51:32PM -0500, Eric Sandeen wrote:

quoted

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.

Ah, so I just checked where the m4/package_urcu.m4 file went -
forgot to re-add it after it rejected on apply. The diff is this:

diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4
new file mode 100644
index 000000000000..9b0dee35d9a1
--- /dev/null
+++ b/m4/package_urcu.m4

@@ -0,0 +1,22 @@
+AC_DEFUN([AC_PACKAGE_NEED_URCU_H],
+  [ AC_CHECK_HEADERS(urcu.h)
+    if test $ac_cv_header_urcu_h = no; then
+       AC_CHECK_HEADERS(urcu.h,, [
+       echo
+       echo 'FATAL ERROR: could not find a valid urcu header.'
+       exit 1])
+    fi
+  ])
+
+AC_DEFUN([AC_PACKAGE_NEED_RCU_INIT],
+  [ AC_MSG_CHECKING([for liburcu])
+    AC_TRY_COMPILE([
+#define _GNU_SOURCE
+#include <urcu.h>
+    ], [
+       rcu_init();
+    ], liburcu=-lurcu
+       AC_MSG_RESULT(yes),
+       AC_MSG_RESULT(no))
+    AC_SUBST(liburcu)
+  ])