Re: [PATCH net v2] napi: fix race inside napi_enable

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 18 Sep 2021 16:52:32 +0800 Xuan Zhuo wrote:
> 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:

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.