Fix: urcu-signal: smp_mb_master() needs registry lock

The signal-based urcu flavor calls smp_mb_master() within the wait_gp()
function. Since commit "Fix: deadlock when thread join is issued in
read-side C.S.", wait_gp() is called without the registry lock held.

Ensure that the registry lock is only released around the wait per se,
not around the call to smp_mb_master(), otherwise we end up iterating on
a non-consistent thread registry in smp_mb_master().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu.c b/urcu.c
index a620b762b4796712bd05efb862de8cb6c53c41fd..a5568bdbd075ed1edc4539dec460c7cce40ee590 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -226,19 +226,27 @@ static void smp_mb_master(void)
 
 /*
  * synchronize_rcu() waiting. Single thread.
+ * Always called with rcu_registry lock held. Releases this lock and
+ * grabs it again. Holds the lock when it returns.
  */
 static void wait_gp(void)
 {
-       /* Read reader_gp before read futex */
+       /*
+        * Read reader_gp before read futex. smp_mb_master() needs to
+        * be called with the rcu registry lock held in RCU_SIGNAL
+        * flavor.
+        */
        smp_mb_master();
+       /* Temporarily unlock the registry lock. */
+       mutex_unlock(&rcu_registry_lock);
        if (uatomic_read(&rcu_gp.futex) != -1)
-               return;
+               goto end;
        while (futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
                        NULL, NULL, 0)) {
                switch (errno) {
                case EWOULDBLOCK:
                        /* Value already changed. */
-                       return;
+                       goto end;
                case EINTR:
                        /* Retry if interrupted by signal. */
                        break;  /* Get out of switch. */
@@ -247,6 +255,11 @@ static void wait_gp(void)
                        urcu_die(errno);
                }
        }
+end:
+       /*
+        * Re-lock the registry lock before the next loop.
+        */
+       mutex_lock(&rcu_registry_lock);
 }
 
 /*
@@ -309,14 +322,19 @@ static void wait_for_readers(struct cds_list_head *input_readers,
                        }
                        break;
                } else {
-                       /* Temporarily unlock the registry lock. */
-                       mutex_unlock(&rcu_registry_lock);
-                       if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
+                       if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+                               /* wait_gp unlocks/locks registry lock. */
                                wait_gp();
-                       else
+                       } else {
+                               /* Temporarily unlock the registry lock. */
+                               mutex_unlock(&rcu_registry_lock);
                                caa_cpu_relax();
-                       /* Re-lock the registry lock before the next loop. */
-                       mutex_lock(&rcu_registry_lock);
+                               /*
+                                * Re-lock the registry lock before the
+                                * next loop.
+                                */
+                               mutex_lock(&rcu_registry_lock);
+                       }
                }
 #else /* #ifndef HAS_INCOHERENT_CACHES */
                /*
@@ -336,16 +354,20 @@ static void wait_for_readers(struct cds_list_head *input_readers,
                                smp_mb_master();
                                wait_gp_loops = 0;
                        }
-                       /* Temporarily unlock the registry lock. */
-                       mutex_unlock(&rcu_registry_lock);
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+                               /* wait_gp unlocks/locks registry lock. */
                                wait_gp();
                                wait_gp_loops++;
                        } else {
+                               /* Temporarily unlock the registry lock. */
+                               mutex_unlock(&rcu_registry_lock);
                                caa_cpu_relax();
+                               /*
+                                * Re-lock the registry lock before the
+                                * next loop.
+                                */
+                               mutex_lock(&rcu_registry_lock);
                        }
-                       /* Re-lock the registry lock before the next loop. */
-                       mutex_lock(&rcu_registry_lock);
                }
 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
        }