[Qemu-devel] [RFC PATCH 1/2] cpus-common: nuke finish_safe_work

[Roman Kagan <rkagan@virtuozzo.com>]

It was introduced in commit b129972c8b41e15b0521895a46fd9c752b68a5e,
with the following motivation:

  Because start_exclusive uses CPU_FOREACH, merge exclusive_lock with
  qemu_cpu_list_lock: together with a call to exclusive_idle (via
  cpu_exec_start/end) in cpu_list_add, this protects exclusive work
  against concurrent CPU addition and removal.

However, it seems to be redundant, because the cpu-exclusive
infrastructure provides suffificent protection against the newly added
CPU starting execution while the cpu-exclusive work is running, and the
aforementioned traversing of the cpu list is protected by
qemu_cpu_list_lock.

Besides, this appears to be the only place where the cpu-exclusive
section is entered with the BQL taken, which has been found to trigger
AB-BA deadlock as follows:

    vCPU thread                             main thread
    -----------                             -----------
async_safe_run_on_cpu(self,
                      async_synic_update)
...                                         [cpu hot-add]
process_queued_cpu_work()
  qemu_mutex_unlock_iothread()
                                            [grab BQL]
  start_exclusive()                         cpu_list_add()
  async_synic_update()                        finish_safe_work()
    qemu_mutex_lock_iothread()                  cpu_exec_start()

So remove it.  This paves the way to establishing a strict nesting rule
of never entering the exclusive section with the BQL taken.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
---
 cpus-common.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/cpus-common.c b/cpus-common.c
index 3ca58c64e8..023cfebfa3 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -69,12 +69,6 @@ static int cpu_get_free_index(void)
     return cpu_index;
 }
 
-static void finish_safe_work(CPUState *cpu)
-{
-    cpu_exec_start(cpu);
-    cpu_exec_end(cpu);
-}
-
 void cpu_list_add(CPUState *cpu)
 {
     qemu_mutex_lock(&qemu_cpu_list_lock);
@@ -86,8 +80,6 @@ void cpu_list_add(CPUState *cpu)
     }
     QTAILQ_INSERT_TAIL_RCU(&cpus, cpu, node);
     qemu_mutex_unlock(&qemu_cpu_list_lock);
-
-    finish_safe_work(cpu);
 }
 
 void cpu_list_remove(CPUState *cpu)
-- 
2.21.0