s3: Avoid a thundering herd in g_lock_unlock

Only notify the first 5 pending lock waiters. This avoids a thundering herd
problem that is really nasty in a cluster. It also makes acquiring a lock a bit
more FIFO, lock waiters are added to the end of the array.

diff --git a/source3/lib/g_lock.c b/source3/lib/g_lock.c
index e2620255d9c00b13f21e5fd995b64a3629dde355..512c0680d9f00992defc58705dbfde4d4e5948c1 100644 (file)
--- a/source3/lib/g_lock.c
+++ b/source3/lib/g_lock.c
@@ -530,13 +530,23 @@ static NTSTATUS g_lock_force_unlock(struct g_lock_ctx *ctx, const char *name,
        }
 
        if ((lock_type & G_LOCK_PENDING) == 0) {
+               int num_wakeups = 0;
+
                /*
-                * We've been the lock holder. Tell all others to retry.
+                * We've been the lock holder. Others to retry. Don't
+                * tell all others to avoid a thundering herd. In case
+                * this leads to a complete stall because we miss some
+                * processes, the loop in g_lock_lock tries at least
+                * once a minute.
                 */
+
                for (i=0; i<num_locks; i++) {
                        if ((locks[i].lock_type & G_LOCK_PENDING) == 0) {
                                continue;
                        }
+                       if (!process_exists(locks[i].pid)) {
+                               continue;
+                       }
 
                        /*
                         * Ping all waiters to retry
@@ -549,6 +559,11 @@ static NTSTATUS g_lock_force_unlock(struct g_lock_ctx *ctx, const char *name,
                                          procid_str(talloc_tos(),
                                                     &locks[i].pid),
                                          nt_errstr(status)));
+                       } else {
+                               num_wakeups += 1;
+                       }
+                       if (num_wakeups > 5) {
+                               break;
                        }
                }
        }