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

[Sukadev Bhattiprolu <sukadev@linux.ibm.com>]

Xuan Zhuo [xuanzhuo@linux.alibaba.com] 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
> }                                |                              |

So its possible that after cpu0 cleared SCHED, cpu1 could have set it
back and we are going to use cmpxchg() to detect and retry right? If so,


>                                  |                              |
> 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>
> ---
>  net/core/dev.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 74fd402d26dd..7ee9fecd3aff 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6923,12 +6923,16 @@ EXPORT_SYMBOL(napi_disable);
>   */
>  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);
> -	if (n->dev->threaded && n->thread)
> -		set_bit(NAPI_STATE_THREADED, &n->state);
> +	unsigned long val, new;
> +
> +	do {
> +		val = READ_ONCE(n->state);
> +		BUG_ON(!test_bit(NAPI_STATE_SCHED, &val));

is this BUG_ON valid/needed? We could have lost the cmpxchg() and
the other thread could have set NAPI_STATE_SCHED?

Sukadev

> +
> +		new = val & ~(NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC);
> +		if (n->dev->threaded && n->thread)
> +			new |= NAPIF_STATE_THREADED;
> +	} while (cmpxchg(&n->state, val, new) != val);
>  }
>  EXPORT_SYMBOL(napi_enable);
>  
> -- 
> 2.31.0