[pve-devel] [PATCH cluster] pmxcfs: protect CPG operations with mutex

[Fabian Grünbichler]

cpg_mcast_joined (and transitively, cpg_join/leave) are not thread-safe.
pmxcfs triggers such operations via FUSE and CPG dispatch callbacks,
which are running in concurrent threads.

accordingly, we need to protect these operations with a mutex, otherwise
they might return CS_OK without actually doing what they were supposed
to do (which in turn can lead to the dfsm taking a wrong turn and
getting stuck in a supposedly short-lived state, blocking access via
FUSE and getting whole clusters fenced).

huge thanks to Alexandre Derumier for providing the initial bug report
and quite a lot of test runs while debugging this issue.

Signed-off-by: Fabian Grünbichler <f.gruenbichler at proxmox.com>
---

Notes:
    we could recycle sync_mutex, but that makes it harder to reason
    about securing all code paths. it also protects non CPG operations
    as part of the sync messsage queue handling, so mixing those up is
    non-ideal.

    @Alexandre: this is a slightly different approach compared to the test
    build from yesterday, so if you want to test this as well it would
    be very welcome :)

 data/src/dfsm.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/data/src/dfsm.c b/data/src/dfsm.c
index 172d877..17a3ba4 100644
--- a/data/src/dfsm.c
+++ b/data/src/dfsm.c
@@ -107,6 +107,7 @@ struct dfsm {
 	cpg_callbacks_t *cpg_callbacks;
 	dfsm_callbacks_t *dfsm_callbacks;
 	cpg_handle_t cpg_handle;
+	GMutex cpg_mutex;
 	struct cpg_name cpg_group_name;
 	uint32_t nodeid;
 	uint32_t pid;
@@ -204,7 +205,9 @@ dfsm_send_message_full(
 	cs_error_t result;
 	int retries = 0;
 loop:
+	g_mutex_lock (&dfsm->cpg_mutex);
 	result = cpg_mcast_joined(dfsm->cpg_handle, CPG_TYPE_AGREED, iov, len);
+	g_mutex_unlock (&dfsm->cpg_mutex);
 	if (retry && result == CS_ERR_TRY_AGAIN) {
 		nanosleep(&tvreq, NULL);
 		++retries;
@@ -1250,7 +1253,9 @@ dfsm_new(
 
 	if (!(dfsm->msg_queue = g_sequence_new(NULL))) 
 		goto err;
-		
+
+	g_mutex_init(&dfsm->cpg_mutex);
+
 	dfsm->log_domain = log_domain;
 	dfsm->data = data;
 	dfsm->mode = DFSM_MODE_START;
@@ -1424,7 +1429,9 @@ dfsm_join(dfsm_t *dfsm)
 	struct timespec tvreq = { .tv_sec = 0, .tv_nsec = 100000000 };
 	int retries = 0;
 loop:
+	g_mutex_lock (&dfsm->cpg_mutex);
 	result = cpg_join(dfsm->cpg_handle, &dfsm->cpg_group_name); 
+	g_mutex_unlock (&dfsm->cpg_mutex);
 	if (result == CS_ERR_TRY_AGAIN) {
 		nanosleep(&tvreq, NULL);
 		++retries;
@@ -1453,7 +1460,9 @@ dfsm_leave (dfsm_t *dfsm)
 	struct timespec tvreq = { .tv_sec = 0, .tv_nsec = 100000000 };
 	int retries = 0;
 loop:
+	g_mutex_lock (&dfsm->cpg_mutex);
 	result = cpg_leave(dfsm->cpg_handle, &dfsm->cpg_group_name);
+	g_mutex_unlock (&dfsm->cpg_mutex);
 	if (result == CS_ERR_TRY_AGAIN) {
 		nanosleep(&tvreq, NULL);
 		++retries;
@@ -1509,6 +1518,8 @@ dfsm_destroy(dfsm_t *dfsm)
 	g_mutex_clear (&dfsm->sync_mutex);
 
 	g_cond_clear (&dfsm->sync_cond);
+
+	g_mutex_clear (&dfsm->cpg_mutex);
  
 	if (dfsm->results)
 		g_hash_table_destroy(dfsm->results);
-- 
2.20.1