Re: [Qemu-devel] [PATCH v3 2/2] QEMUBH: make AioContext's bh re-entrant

[liu ping fan]

On Thu, Jun 20, 2013 at 4:16 PM, Paolo Bonzini <address@hidden> wrote:
> Il 20/06/2013 09:39, Stefan Hajnoczi ha scritto:
>> qemu_bh_cancel() and qemu_bh_delete() are not modified by this patch.
>>
>> It seems that calling them from a thread is a little risky because there
>> is no guarantee that the BH is no longer invoked after a thread calls
>> these functions.
>>
>> I think that's worth a comment or do you want them to take the lock so
>> they become safe?
>
> Taking the lock wouldn't help.  The invoking loop of aio_bh_poll runs
> lockless.  I think a comment is better.
>
> qemu_bh_cancel is inherently not thread-safe, there's not much you can
> do about it.
>
> qemu_bh_delete is safe as long as you wait for the bottom half to stop
> before deleting the containing object.  Once we have RCU, deletion of
> QOM objects will be RCU-protected.  Hence, a simple way could be to put
> the first part of aio_bh_poll() within rcu_read_lock/unlock.
>
In fact, I have some idea about this,  introduce another member -
Object for QEMUBH which will be refereed in cb, then we leave anything
to refcnt mechanism.
For qemu_bh_cancel(), I do not figure out whether it is important or
not to sync with caller.

diff --git a/async.c b/async.c
index 4b17eb7..60c35a1 100644
--- a/async.c
+++ b/async.c
@@ -61,6 +61,7 @@ int aio_bh_poll(AioContext *ctx)
 {
     QEMUBH *bh, **bhp, *next;
     int ret;
+    int sched;

 {
     QEMUBH *bh, **bhp, *next;
     int ret;
+    int sched;

     ctx->walking_bh++;

@@ -69,8 +70,10 @@ int aio_bh_poll(AioContext *ctx)
         /* Make sure fetching bh before accessing its members */
         smp_read_barrier_depends();
         next = bh->next;
-        if (!bh->deleted && bh->scheduled) {
-            bh->scheduled = 0;
+        sched = 0;
+        atomic_xchg(&bh->scheduled, sched);
+        if (!bh->deleted && sched) {
+            //bh->scheduled = 0;
             if (!bh->idle)
                 ret = 1;
             bh->idle = 0;
@@ -79,6 +82,9 @@ int aio_bh_poll(AioContext *ctx)
              */
             smp_rmb();
             bh->cb(bh->opaque);
+            if (bh->obj) {
+                object_unref(bh->obj);
+            }
         }
     }

@@ -105,8 +111,12 @@ int aio_bh_poll(AioContext *ctx)

 void qemu_bh_schedule_idle(QEMUBH *bh)
 {
-    if (bh->scheduled)
+    int sched = 1;
+
+    atomic_xchg( &bh->scheduled, sched);
+    if (sched) {
         return;
+    }
     /* Make sure any writes that are needed by the callback are done
      * before the locations are read in the aio_bh_poll.
      */
@@ -117,25 +127,46 @@ void qemu_bh_schedule_idle(QEMUBH *bh)

 void qemu_bh_schedule(QEMUBH *bh)
 {
-    if (bh->scheduled)
+    int sched = 1;
+
+    atomic_xchg( &bh->scheduled, sched);
+    if (sched) {
         return;
+    }
     /* Make sure any writes that are needed by the callback are done
      * before the locations are read in the aio_bh_poll.
      */
     smp_wmb();
     bh->scheduled = 1;
+    if (bh->obj) {
+        object_ref(bh->obj);
+    }
     bh->idle = 0;
     aio_notify(bh->ctx);
 }

 void qemu_bh_cancel(QEMUBH *bh)
 {
-    bh->scheduled = 0;
+    int sched = 0;
+
+    atomic_xchg( &bh->scheduled, sched);
+    if (sched) {
+        if (bh->obj) {
+            object_ref(bh->obj);
+        }
+    }
 }

 void qemu_bh_delete(QEMUBH *bh)
 {
-    bh->scheduled = 0;
+    int sched = 0;
+
+    atomic_xchg( &bh->scheduled, sched);
+    if (sched) {
+        if (bh->obj) {
+            object_ref(bh->obj);
+        }
+    }
     bh->deleted = 1;
 }

Regards,
Pingfan
>> The other thing I'm unclear on is the ->idle assignment followed
>> immediately by a ->scheduled assignment.  Without memory barriers
>> aio_bh_poll() isn't guaranteed to get an ordered view of these updates:
>> it may see an idle BH as a regular scheduled BH because ->idle is still
>> 0.
>
> Right.  You need to order ->idle writes before ->scheduled writes, and
> add memory barriers, or alternatively use two bits in ->scheduled so
> that you can assign both atomically.
>
> Paolo