[Qemu-devel] [RFC 1/2] linux-user: Don't leak cpus on thread exit

[David Gibson <david@gibson.dropbear.id.au>]

Currently linux-user does not correctly clean up CPU instances properly
when running a threaded binary.

On thread exit, do_syscall() removes the thread's CPU from the cpus list
and calls object_unref().  However, the CPU still is still referenced from
the QOM tree.  To correctly clean up we need to object_unparent() to remove
the CPU from the QOM tree, then object_unref() to release the final
reference we're holding.

Once this is done, the explicit remove from the cpus list is no longer
necessary, since that's done automatically in the CPU unrealize path.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 linux-user/syscall.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

I believe most full system targets also "leak" cpus in the same way,
except that since they don't support cpu hot unplug the cpus never
would have been disposed anyway.  I'll look into fixing that another
time.

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 8bf6205..dd91791 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -6823,10 +6823,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         if (CPU_NEXT(first_cpu)) {
             TaskState *ts;
 
-            cpu_list_lock();
-            /* Remove the CPU from the list.  */
-            QTAILQ_REMOVE(&cpus, cpu, node);
-            cpu_list_unlock();
+            object_unparent(OBJECT(cpu)); /* Remove from QOM */
             ts = cpu->opaque;
             if (ts->child_tidptr) {
                 put_user_u32(0, ts->child_tidptr);
@@ -6834,7 +6831,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
                           NULL, NULL, 0);
             }
             thread_cpu = NULL;
-            object_unref(OBJECT(cpu));
+            object_unref(OBJECT(cpu)); /* Remove the last ref we're holding */
             g_free(ts);
             rcu_unregister_thread();
             pthread_exit(NULL);
-- 
2.7.4