From e533f45d7dd85d6514de3f7a433f7dc4313e8f62 Mon Sep 17 00:00:00 2001 From: Roman Kagan Date: Thu, 23 May 2019 10:54:48 +0000 Subject: [PATCH] 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 Message-Id: <20190523105440.27045-2-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)