rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

PATCH Superseded Wen Congyang 2 participants 5 messages Jul 27, 2015

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Wen Congyang

author

2015-07-27 02:24 UTC #0 lore raw

PATCH 1/1 rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

If rcu_(un)register_thread() is called together with synchronize_rcu(), it will wait for the synchronize_rcu() to finish. But when synchronize_rcu() waits for some events, we can modify the list registry.
We also use the lock rcu_gp_lock to assume that synchronize_rcu() isn't executed in more than one thread at the same time. Add a new mutex lock rcu_sync_lock to assume it and rename rcu_gp_lock to rcu_registry_lock. Release rcu_registry_lock when synchronize_rcu() waits for some events.

Signed-off-by Wen Congyang <[email protected]>

---
 util/rcu.c | 36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

util/rcu.c +23 ~~-13~~

index cdcad67..18c5792 100644
@@ -47,7 +47,8 @@
 unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
 
 QemuEvent rcu_gp_event;
-static QemuMutex rcu_gp_lock;
+static QemuMutex rcu_registry_lock;
+static QemuMutex rcu_sync_lock;
 
 /*
  * Check whether a quiescent state was crossed between the beginning of

@@ -66,7 +67,7 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
  */
 __thread struct rcu_reader_data rcu_reader;
 
-/* Protected by rcu_gp_lock.  */
+/* Protected by rcu_registry_lock.  */
 typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
 static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);

@@ -114,10 +115,14 @@ static void wait_for_readers(void)
             break;
         }
 
-        /* Wait for one thread to report a quiescent state and
-         * try again.
+        /* Wait for one thread to report a quiescent state and try again.
+         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
+         * wait too much time. Note: rcu_unregister_thread() may remove
+         * the node from qsreaders. That's a bit tricky, but it should work.
          */
+        qemu_mutex_unlock(&rcu_registry_lock);
         qemu_event_wait(&rcu_gp_event);
+        qemu_mutex_lock(&rcu_registry_lock);
     }
 
     /* put back the reader list in the registry */

@@ -126,7 +131,8 @@ static void wait_for_readers(void)
 
 void synchronize_rcu(void)
 {
-    qemu_mutex_lock(&rcu_gp_lock);
+    qemu_mutex_lock(&rcu_sync_lock);
+    qemu_mutex_lock(&rcu_registry_lock);
 
     if (!QLIST_EMPTY(&registry)) {
         /* In either case, the atomic_mb_set below blocks stores that free

@@ -149,7 +155,8 @@ void synchronize_rcu(void)
         wait_for_readers();
     }
 
-    qemu_mutex_unlock(&rcu_gp_lock);
+    qemu_mutex_unlock(&rcu_registry_lock);
+    qemu_mutex_unlock(&rcu_sync_lock);
 }

@@ -273,23 +280,24 @@ void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
 void rcu_register_thread(void)
 {
     assert(rcu_reader.ctr == 0);
-    qemu_mutex_lock(&rcu_gp_lock);
+    qemu_mutex_lock(&rcu_registry_lock);
     QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
-    qemu_mutex_unlock(&rcu_gp_lock);
+    qemu_mutex_unlock(&rcu_registry_lock);
 }
 
 void rcu_unregister_thread(void)
 {
-    qemu_mutex_lock(&rcu_gp_lock);
+    qemu_mutex_lock(&rcu_registry_lock);
     QLIST_REMOVE(&rcu_reader, node);
-    qemu_mutex_unlock(&rcu_gp_lock);
+    qemu_mutex_unlock(&rcu_registry_lock);
 }
 
 static void rcu_init_complete(void)
 {
     QemuThread thread;
 
-    qemu_mutex_init(&rcu_gp_lock);
+    qemu_mutex_init(&rcu_registry_lock);
+    qemu_mutex_init(&rcu_sync_lock);
     qemu_event_init(&rcu_gp_event, true);
 
     qemu_event_init(&rcu_call_ready_event, false);

@@ -306,12 +314,14 @@ static void rcu_init_complete(void)
 #ifdef CONFIG_POSIX
 static void rcu_init_lock(void)
 {
-    qemu_mutex_lock(&rcu_gp_lock);
+    qemu_mutex_lock(&rcu_sync_lock);
+    qemu_mutex_lock(&rcu_registry_lock);
 }
 
 static void rcu_init_unlock(void)
 {
-    qemu_mutex_unlock(&rcu_gp_lock);
+    qemu_mutex_unlock(&rcu_registry_lock);
+    qemu_mutex_unlock(&rcu_sync_lock);
 }
 #endif

Signature · 1 line

2.4.3

Paolo Bonzini

2015-07-27 10:17 UTC #1 lore raw

util/rcu.c static void wait_for_readers(void) · line 121

Quoted 4 lines · Wen Congyang

util/rcu.c +4 -0

+        /* Wait for one thread to report a quiescent state and try again.
+         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
+         * wait too much time. Note: rcu_unregister_thread() may remove
+         * the node from qsreaders. That's a bit tricky, but it should work.

"It should work" is a bit optimistic. :D

Does this description look okay?

/* Wait for one thread to report a quiescent state and try again.
* Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
* wait too much time.
*
* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* process the new thread or set ->waiting for it. Hence, this can
* at worst cause synchronize_rcu() to wait for longer.
*
* rcu_unregister_thread() may remove nodes from &qsreaders instead
* of &registry if it runs during qemu_event_wait. That's okay;
* the node then will not be added back to &registry by QLIST_SWAP
* below. The invariant is that the node is part of one list when
* rcu_registry_lock is released.
*/

Paolo

Wen Congyang

author

2015-07-27 10:44 UTC #2 lore raw

Quoted 12 lines · Paolo Bonzini

util/rcu.c static void wait_for_readers(void) · line 121
Quoted 4 lines · Wen Congyang
util/rcu.c +4 -0
+        /* Wait for one thread to report a quiescent state and try again.
+         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
+         * wait too much time. Note: rcu_unregister_thread() may remove
+         * the node from qsreaders. That's a bit tricky, but it should work.
"It should work" is a bit optimistic. :D
Does this description look okay?

Quoted 10 lines

/* Wait for one thread to report a quiescent state and try again.
* Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
* wait too much time.
*
* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* process the new thread or set ->waiting for it. Hence, this can
* at worst cause synchronize_rcu() to wait for longer.

I don't understand this. The next iteration of the loop will move the new thread's rcu_reader from registry to qsreaders even if we call rcu_read_lock() in the new thread. Because rcu_gp_ongoing() will return false.

Thanks
Wen Congyang

Quoted 11 lines

*
* rcu_unregister_thread() may remove nodes from &qsreaders instead
* of &registry if it runs during qemu_event_wait. That's okay;
* the node then will not be added back to &registry by QLIST_SWAP
* below. The invariant is that the node is part of one list when
* rcu_registry_lock is released.
*/
Paolo
.

Paolo Bonzini

2015-07-27 10:52 UTC #3 lore raw

Quoted 9 lines · Wen Congyang

Quoted 6 lines
* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* process the new thread or set ->waiting for it. Hence, this can
* at worst cause synchronize_rcu() to wait for longer.
I don't understand this. The next iteration of the loop will move the new thread's rcu_reader from registry to qsreaders even if we call rcu_read_lock() in the new thread. Because rcu_gp_ongoing() will return false.

You're right. This proves that a comment was necessary! :)

Second try:

* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* move the new thread's rcu_reader from &registry to &qsreaders,
* because rcu_gp_ongoing() will return false.

Paolo

Wen Congyang

author

2015-07-27 10:54 UTC #4 lore raw

Quoted 9 lines · Paolo Bonzini

Quoted 6 lines · Wen Congyang
Quoted 6 lines
* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* process the new thread or set ->waiting for it. Hence, this can
* at worst cause synchronize_rcu() to wait for longer.

Quoted 5 lines

Quoted 3 lines
I don't understand this. The next iteration of the loop will move the new thread's rcu_reader from registry to qsreaders even if we call rcu_read_lock() in the new thread. Because rcu_gp_ongoing() will return false.
You're right. This proves that a comment was necessary! :)

Yes, I agree with it.

Quoted 9 lines

Second try:
* rcu_register_thread() may add nodes to &registry; it will not
* wake up synchronize_rcu, but that is okay because at least another
* thread must exit its RCU read-side critical section before
* synchronize_rcu is done. The next iteration of the loop will
* move the new thread's rcu_reader from &registry to &qsreaders,
* because rcu_gp_ongoing() will return false.

I will update the comment and send it again.

Thanks
Wen Congyang

Quoted 4 lines

Paolo
.