Re: [PATCH] rps: Handle double list_add at __napi_schedule

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2015-06-15 at 21:46 +0000, subashab@codeaurora.org wrote:
> When NAPI_STATE_SCHED state is not set, enqueue_to_backlog()
> will queue an IPI and add the backlog queue to the poll list. A packet
> added by RPS onto the core could also add the NAPI backlog struct to the
> poll list. This double addition to the list causes a crash -
> 
> 2920.540304:   <2> list_add double add: new=ffffffc076ed2930,
> prev=ffffffc076ed2930, next=ffffffc076ed2850.
>    [<ffffffc000460dd4>] __list_add+0xcc/0xf0
> 2921.064962:   <2> [<ffffffc000b44880>] rps_trigger_softirq+0x1c/0x40
> 2921.070779:   <2> [<ffffffc000284a14>]
> generic_smp_call_function_single_interrupt+0xe8/0x12c
> 2921.078678:   <2> [<ffffffc00020d9ac>] handle_IPI+0x8c/0x1ec
> 2921.083796:   <2> [<ffffffc000200714>] gic_handle_irq+0x94/0xb0
> 
> Fix this race for double addition to list by checking the NAPI state.
> 
> Acked-by: Sharat Masetty <smasetty@qti.qualcomm.com>
> Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 6f561de..57d6d39 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3225,7 +3225,8 @@ static void rps_trigger_softirq(void *data)
>  {
>         struct softnet_data *sd = data;
> 
> -       ____napi_schedule(sd, &sd->backlog);
> +       if (!test_bit(NAPI_STATE_SCHED, &sd->backlog.state))
> +               ____napi_schedule(sd, &sd->backlog);
>         sd->received_rps++;
>  }
> 

I can not believe how many times you tried to send RPS patches.

I do not see how this condition triggers. This code path is run billions
of time per ms on our hosts and never got a single crash.

Please describe where is the race condition you want to fix.

Your test is racy by definition.