[Libguestfs] [PATCH libnbd 4/4] lib: Atomically update h->state when leaving the locked region.
Richard W.M. Jones
rjones at redhat.com
Wed Jun 5 11:15:37 UTC 2019
Split h->state into:
- h->state = the state on entry to the locked region
- h->next_state = the current state and what the "publicly visible"
state will become when we leave the locked region
Some calls to get_state become calls to get_next_state depending on
which of these they are trying to read. Calls to set_state become
set_next_state because that is what gets updated.
When we leave the locked region we update h->state.
The purpose of this patch is to make it easier to reason about the
state in lockless code.
---
generator/generator | 23 +++++++++++++----------
lib/connect.c | 10 +++++-----
lib/disconnect.c | 8 ++++----
lib/handle.c | 2 +-
lib/internal.h | 15 +++++++++++++--
lib/rw.c | 4 ++--
6 files changed, 38 insertions(+), 24 deletions(-)
diff --git a/generator/generator b/generator/generator
index a3fbea9..d198007 100755
--- a/generator/generator
+++ b/generator/generator
@@ -2415,11 +2415,11 @@ let generate_lib_states_c () =
pr " enum state next_state = %s;\n" state_enum;
pr "\n";
pr " r = _enter_%s (h, &next_state, blocked);\n" state_enum;
- pr " if (get_state (h) != next_state) {\n";
+ pr " if (get_next_state (h) != next_state) {\n";
pr " debug (h, \"transition: %%s -> %%s\",\n";
pr " \"%s\",\n" display_name;
pr " nbd_internal_state_short_string (next_state));\n";
- pr " set_state (h, next_state);\n";
+ pr " set_next_state (h, next_state);\n";
pr " }\n";
pr " return r;\n";
pr "}\n";
@@ -2434,7 +2434,7 @@ let generate_lib_states_c () =
pr " bool blocked;\n";
pr "\n";
pr " /* Validate and handle the external event. */\n";
- pr " switch (get_state (h))\n";
+ pr " switch (get_next_state (h))\n";
pr " {\n";
List.iter (
fun ({ parsed = { display_name; state_enum; events } } as state) ->
@@ -2446,7 +2446,7 @@ let generate_lib_states_c () =
fun (e, next_state) ->
pr " case %s:\n" (c_string_of_external_event e);
if state != next_state then (
- pr " set_state (h, %s);\n" next_state.parsed.state_enum;
+ pr " set_next_state (h, %s);\n" next_state.parsed.state_enum;
pr " debug (h, \"event %%s: %%s -> %%s\",\n";
pr " \"%s\", \"%s\", \"%s\");\n"
(string_of_external_event e)
@@ -2462,7 +2462,7 @@ let generate_lib_states_c () =
pr " }\n";
pr "\n";
pr " set_error (EINVAL, \"external event %%d is invalid in state %%s\",\n";
- pr " ev, nbd_internal_state_short_string (get_state (h)));\n";
+ pr " ev, nbd_internal_state_short_string (get_next_state (h)));\n";
pr " return -1;\n";
pr "\n";
pr " ok:\n";
@@ -2470,7 +2470,7 @@ let generate_lib_states_c () =
pr " blocked = true;\n";
pr "\n";
pr " /* Run a single step. */\n";
- pr " switch (get_state (h))\n";
+ pr " switch (get_next_state (h))\n";
pr " {\n";
List.iter (
fun { parsed = { state_enum } } ->
@@ -2496,7 +2496,7 @@ let generate_lib_states_c () =
pr "{\n";
pr " int r = 0;\n";
pr "\n";
- pr " switch (get_state (h))\n";
+ pr " switch (get_next_state (h))\n";
pr " {\n";
List.iter (
fun ({ parsed = { state_enum; events } }) ->
@@ -2542,7 +2542,7 @@ let generate_lib_states_c () =
pr "const char *\n";
pr "nbd_unlocked_connection_state (struct nbd_handle *h)\n";
pr "{\n";
- pr " switch (get_state (h))\n";
+ pr " switch (get_next_state (h))\n";
pr " {\n";
List.iter (
fun ({ comment; parsed = { display_name; state_enum } }) ->
@@ -2866,8 +2866,11 @@ let generate_lib_api_c () =
let argnames = List.flatten (List.map name_of_arg args) in
List.iter (pr ", %s") argnames;
pr ");\n";
- if is_locked then
- pr " pthread_mutex_unlock (&h->lock);\n";
+ if is_locked then (
+ pr " if (h->state != h->next_state)\n";
+ pr " h->state = h->next_state;\n";
+ pr " pthread_mutex_unlock (&h->lock);\n"
+ );
pr " return ret;\n";
pr "}\n";
pr "\n";
diff --git a/lib/connect.c b/lib/connect.c
index b889f80..4e3141f 100644
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -38,16 +38,16 @@
static int
error_unless_ready (struct nbd_handle *h)
{
- if (nbd_internal_is_state_ready (get_state (h)))
+ if (nbd_internal_is_state_ready (get_next_state (h)))
return 0;
/* Why did it fail? */
- if (nbd_internal_is_state_closed (get_state (h))) {
+ if (nbd_internal_is_state_closed (get_next_state (h))) {
set_error (0, "connection is closed");
return -1;
}
- if (nbd_internal_is_state_dead (get_state (h)))
+ if (nbd_internal_is_state_dead (get_next_state (h)))
/* Don't set the error here, keep the error set when
* the connection died.
*/
@@ -55,14 +55,14 @@ error_unless_ready (struct nbd_handle *h)
/* Should probably never happen. */
set_error (0, "connection in an unexpected state (%s)",
- nbd_internal_state_short_string (get_state (h)));
+ nbd_internal_state_short_string (get_next_state (h)));
return -1;
}
static int
wait_until_connected (struct nbd_handle *h)
{
- while (nbd_internal_is_state_connecting (get_state (h))) {
+ while (nbd_internal_is_state_connecting (get_next_state (h))) {
if (nbd_unlocked_poll (h, -1) == -1)
return -1;
}
diff --git a/lib/disconnect.c b/lib/disconnect.c
index 423edaf..95e9a37 100644
--- a/lib/disconnect.c
+++ b/lib/disconnect.c
@@ -29,14 +29,14 @@
int
nbd_unlocked_shutdown (struct nbd_handle *h)
{
- if (nbd_internal_is_state_ready (get_state (h)) ||
- nbd_internal_is_state_processing (get_state (h))) {
+ if (nbd_internal_is_state_ready (get_next_state (h)) ||
+ nbd_internal_is_state_processing (get_next_state (h))) {
if (nbd_unlocked_aio_disconnect (h, 0) == -1)
return -1;
}
- while (!nbd_internal_is_state_closed (get_state (h)) &&
- !nbd_internal_is_state_dead (get_state (h))) {
+ while (!nbd_internal_is_state_closed (get_next_state (h)) &&
+ !nbd_internal_is_state_dead (get_next_state (h))) {
if (nbd_unlocked_poll (h, -1) == -1)
return -1;
}
diff --git a/lib/handle.c b/lib/handle.c
index cc311ba..8bf5280 100644
--- a/lib/handle.c
+++ b/lib/handle.c
@@ -57,7 +57,7 @@ nbd_create (void)
s = getenv ("LIBNBD_DEBUG");
h->debug = s && strcmp (s, "1") == 0;
- h->state = STATE_START;
+ h->state = h->next_state = STATE_START;
h->pid = -1;
h->export_name = strdup ("");
diff --git a/lib/internal.h b/lib/internal.h
index 7290247..1bef1c2 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -80,7 +80,17 @@ struct nbd_handle {
/* Linked list of close callbacks. */
struct close_callback *close_callbacks;
- _Atomic enum state state; /* State machine. */
+ /* State machine.
+ *
+ * The actual current state is ‘next_state’. ‘state’ is updated
+ * before we release the lock.
+ *
+ * Note don't access these fields directly, use the SET_NEXT_STATE
+ * macro in generator/states* code, or the set_next_state,
+ * get_next_state and get_state macros in regular code.
+ */
+ _Atomic enum state state;
+ enum state next_state;
bool structured_replies; /* If we negotiated NBD_OPT_STRUCTURED_REPLY */
@@ -291,7 +301,8 @@ extern const char *nbd_internal_state_short_string (enum state state);
extern enum state_group nbd_internal_state_group (enum state state);
extern enum state_group nbd_internal_state_group_parent (enum state_group group);
-#define set_state(h,next_state) ((h)->state) = (next_state)
+#define set_next_state(h,_next_state) ((h)->next_state) = (_next_state)
+#define get_next_state(h) ((h)->next_state)
#define get_state(h) ((h)->state)
/* utils.c */
diff --git a/lib/rw.c b/lib/rw.c
index b38d95b..ad9c8a0 100644
--- a/lib/rw.c
+++ b/lib/rw.c
@@ -201,7 +201,7 @@ nbd_internal_command_common (struct nbd_handle *h,
* be handled automatically on a future cycle around to READY.
*/
if (h->cmds_to_issue != NULL) {
- assert (nbd_internal_is_state_processing (get_state (h)));
+ assert (nbd_internal_is_state_processing (get_next_state (h)));
prev_cmd = h->cmds_to_issue;
while (prev_cmd->next)
prev_cmd = prev_cmd->next;
@@ -209,7 +209,7 @@ nbd_internal_command_common (struct nbd_handle *h,
}
else {
h->cmds_to_issue = cmd;
- if (nbd_internal_is_state_ready (get_state (h)) &&
+ if (nbd_internal_is_state_ready (get_next_state (h)) &&
nbd_internal_run (h, cmd_issue) == -1)
return -1;
}
--
2.21.0
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