cpus-common: nuke finish_safe_work
It was introduced in commit ab129972c8b41e15b0521895a46fd9c752b68a5e,
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>
Message-Id: <20190523105440.27045-2-rkagan@virtuozzo.com>
diff --git a/cpus-common.c b/cpus-common.c
index 3ca58c6..023cfeb 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -69,12 +69,6 @@
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 @@
}
QTAILQ_INSERT_TAIL_RCU(&cpus, cpu, node);
qemu_mutex_unlock(&qemu_cpu_list_lock);
-
- finish_safe_work(cpu);
}
void cpu_list_remove(CPUState *cpu)
