Re: [PATCH net v2] napi: fix race inside napi_enable
From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Date: 2021-09-22 06:48:18
On Mon, 20 Sep 2021 12:20:24 -0700, Jakub Kicinski [off-list ref] wrote:
On Sat, 18 Sep 2021 16:52:32 +0800 Xuan Zhuo wrote:quoted
The process will cause napi.state to contain NAPI_STATE_SCHED and not in the poll_list, which will cause napi_disable() to get stuck. The prefix "NAPI_STATE_" is removed in the figure below, and NAPI_STATE_HASHED is ignored in napi.state. CPU0 | CPU1 | napi.state =============================================================================== napi_disable() | | SCHED | NPSVC napi_enable() | | { | | smp_mb__before_atomic(); | | clear_bit(SCHED, &n->state); | | NPSVC | napi_schedule_prep() | SCHED | NPSVC | napi_poll() | | napi_complete_done() | | { | | if (n->state & (NPSVC | | (1) | _BUSY_POLL))) | | return false; | | ................ | | } | SCHED | NPSVC | | clear_bit(NPSVC, &n->state); | | SCHED } | | | | napi_schedule_prep() | | SCHED | MISSED (2) (1) Here return direct. Because of NAPI_STATE_NPSVC exists. (2) NAPI_STATE_SCHED exists. So not add napi.poll_list to sd->poll_list Since NAPI_STATE_SCHED already exists and napi is not in the sd->poll_list queue, NAPI_STATE_SCHED cannot be cleared and will always exist. 1. This will cause this queue to no longer receive packets. 2. If you encounter napi_disable under the protection of rtnl_lock, it will cause the entire rtnl_lock to be locked, affecting the overall system. This patch uses cmpxchg to implement napi_enable(), which ensures that there will be no race due to the separation of clear two bits. Fixes: 2d8bff12699abc ("netpoll: Close race condition between poll_one_napi and napi_disable") Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com> Reviewed-by: Dust Li <dust.li@linux.alibaba.com>Why don't you just invert the order of clearing the bits:
I think it should be an atomic operation. The original two-step clear itself is problematic. So from this perspective, it is not just a solution to this problem. Thanks.
quoted hunk ↗ jump to hunk
diff --git a/net/core/dev.c b/net/core/dev.c index a796754f75cc..706eca8112c1 100644 --- a/net/core/dev.c +++ b/net/core/dev.c@@ -6953,8 +6953,8 @@ void napi_enable(struct napi_struct *n) { BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state)); smp_mb__before_atomic(); - clear_bit(NAPI_STATE_SCHED, &n->state); clear_bit(NAPI_STATE_NPSVC, &n->state); + clear_bit(NAPI_STATE_SCHED, &n->state); if (n->dev->threaded && n->thread) set_bit(NAPI_STATE_THREADED, &n->state); }That's simpler and symmetric with the disable path.