MINOR: fd: add functions to manipulate the FD's tgid

The FD's tgid is refcounted and must be atomically manipulated. Function
fd_grab_tgid() will increase the refcount but only if the tgid matches the
one in argument (likely the current one). fd_claim_tgid() will be used to
self-assign the tgid after waiting for its refcount to reach zero.
fd_drop_tgid() will be used to drop a temporarily held tgid. All of these
are needed to prevent an FD from being reassigned to another group, either
when inspecting/modifying the running_mask, or when checking for updates,
in order to be certain that the mask being seen corresponds to the desired
group. Note that once at least one bit is set in the running mask of an
active FD, it cannot be closed, thus not migrated, thus the reference does
not need to be held long.

include/haproxy/fd.h

@@ -320,6 +320,73 @@ static forceinline int fd_tgid(int fd)
 	return _HA_ATOMIC_LOAD(&fdtab[fd].refc_tgid) & 0xFFFF;
 }
 
+/* Release a tgid previously taken by fd_grab_tgid() */
+static forceinline void fd_drop_tgid(int fd)
+{
+	HA_ATOMIC_SUB(&fdtab[fd].refc_tgid, 0x10000);
+}
+
+/* Grab a reference to the FD's TGID, and return the tgid. Note that a TGID of
+ * zero indicates the FD was closed, thus also fails (i.e. no need to drop it).
+ * On non-zero (success), the caller must release it using fd_drop_tgid().
+ */
+static inline uint fd_take_tgid(int fd)
+{
+	uint old;
+
+	old = _HA_ATOMIC_FETCH_ADD(&fdtab[fd].refc_tgid, 0x10000) & 0xffff;
+	if (likely(old))
+		return old;
+	HA_ATOMIC_SUB(&fdtab[fd].refc_tgid, 0x10000);
+	return 0;
+}
+
+/* Reset a tgid without affecting the refcount */
+static forceinline void fd_reset_tgid(int fd)
+{
+	HA_ATOMIC_AND(&fdtab[fd].refc_tgid, 0xffff0000U);
+}
+
+/* Try to grab a reference to the FD's TGID, but only if it matches the
+ * requested one (i.e. it succeeds with TGID refcnt held, or fails). Note that
+ * a TGID of zero indicates the FD was closed, thus also fails. It returns
+ * non-zero on success, in which case the caller must then release it using
+ * fd_drop_tgid(), or zero on failure. The function is optimized for use
+ * when it's likely that the tgid matches the desired one as it's by far
+ * the most common.
+ */
+static inline uint fd_grab_tgid(int fd, uint desired_tgid)
+{
+	uint old;
+
+	old = _HA_ATOMIC_FETCH_ADD(&fdtab[fd].refc_tgid, 0x10000) & 0xffff;
+	if (likely(old == desired_tgid))
+		return 1;
+	HA_ATOMIC_SUB(&fdtab[fd].refc_tgid, 0x10000);
+	return 0;
+}
+
+/* Set the FD's TGID to the new value with a refcount of 1, waiting for the
+ * current refcount to become 0, to cover the rare possibly that a late
+ * competing thread would be touching the tgid or the running mask in parallel.
+ * The caller must call fd_drop_tgid() once done.
+ */
+static inline void fd_claim_tgid(int fd, uint desired_tgid)
+{
+	uint old;
+
+	BUG_ON(!desired_tgid);
+
+	desired_tgid += 0x10000; // refcount=1
+	old = desired_tgid;
+	while (1) {
+		old &= 0xffff;
+		if (_HA_ATOMIC_CAS(&fdtab[fd].refc_tgid, &old, desired_tgid))
+			break;
+		__ha_cpu_relax();
+	}
+}
+
 /* remove tid_bit from the fd's running mask and returns the bits that remain
  * after the atomic operation.
  */