Use atomic open file reference counts

- Use relaxed increment under the map bucket lock when duplicating file handles.
- Use acq_rel decrement when releasing and checking for final references.
- Load moved ref_count with relaxed ordering during map insertion.

src/fuse_open.cpp

@@ -367,10 +367,19 @@ _open(const fuse_req_ctx_t *ctx_,
 
       // The ref increment keeps fuse_release.cpp from erasing and
       // freeing the canonical entry while we duplicate its fd.
+      //
+      // memory_order_relaxed is sufficient for the increment. The
+      // concurrent_flat_map bucket lock held inside visit() already
+      // provides the happens-before relationship with other mutators
+      // of the same key (the release() erase_if() callback and other
+      // open() visitors.) The lock's release/acquire semantics
+      // publish the ref_count update to subsequent observers of the
+      // same entry, so we only need atomicity from the fetch_add
+      // itself.
       of.visit(ctx_->nodeid,
                [&](auto &v_)
                {
-                 v_.second.ref_count++;
+                 v_.second.ref_count.fetch_add(1,std::memory_order_relaxed);
                  fi = v_.second.fi;
                  backing_id = v_.second.backing_id;
                });

src/fuse_release.cpp

@@ -81,11 +81,21 @@ FUSE::release(cu64             nodeid_,
   FileInfo *canonical_fi  = nullptr;
   FileInfo *fh_fi_to_free = nullptr;
 
+  // memory_order_acq_rel on the decrement gives us the canonical
+  // refcount-publication pattern:
+  //   - release: prior writes to the OpenFile by this thread are
+  //     visible to whichever thread observes the final 0.
+  //   - acquire: the thread that drops the last ref acquires every
+  //     prior ref holder's writes before tearing down the entry.
+  // The concurrent_flat_map bucket lock already serializes concurrent
+  // visitors of this key so the acq_rel is good for any observer that
+  // might examine ref_count outside a visit().
   of.erase_if(nodeid_,
               [&](auto &v_)
               {
-                v_.second.ref_count--;
-                if(v_.second.ref_count > 0)
+                const int prev =
+                  v_.second.ref_count.fetch_sub(1,std::memory_order_acq_rel);
+                if(prev > 1)
                   {
                     if(fi_ != v_.second.fi)
                       fh_fi_to_free = fi_;

src/state.hpp

@@ -103,8 +103,12 @@ public:
     {
     }
 
+    // Move constructor runs during initial insertion into the
+    // concurrent_flat_map before the new entry is reachable by the
+    // other threads. There is no concurrent observer of the moved
+    // value at this point so relaxed load is sufficient.
     OpenFile(OpenFile &&o_) noexcept
-      : ref_count(o_.ref_count.load()),
+      : ref_count(o_.ref_count.load(std::memory_order_relaxed)),
         backing_id(o_.backing_id),
         fi(o_.fi)
     {