On Wed, Feb 2, 2022 at 4:28 AM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> The preempt_disable() and rcu_disable() section was introduced in commit
> bbbe211c295ff ("net: rcu lock and preempt disable missing around generic xdp")
>
> The backtrace shows that bottom halves were disabled and so the usage of
> smp_processor_id() would not trigger a warning.
> The "suspicious RCU usage" warning was triggered because
> rcu_dereference() was not used in rcu_read_lock() section (only
> rcu_read_lock_bh()). A rcu_read_lock() is sufficient.
>
> Remove the preempt_disable() statement which is not needed.
I am confused by this changelog/analysis of yours.
According to git blame, you are reverting this patch.
commit cece1945bffcf1a823cdfa36669beae118419351
Author: Changli Gao <xiaosuo@xxxxxxxxx>
Date: Sat Aug 7 20:35:43 2010 -0700
net: disable preemption before call smp_processor_id()
Although netif_rx() isn't expected to be called in process context with
preemption enabled, it'd better handle this case. And this is why get_cpu()
is used in the non-RPS #ifdef branch. If tree RCU is selected,
rcu_read_lock() won't disable preemption, so preempt_disable() should be
called explictly.
Signed-off-by: Changli Gao <xiaosuo@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
But I am not sure we can.
Here is the code in larger context:
#ifdef CONFIG_RPS
if (static_branch_unlikely(&rps_needed)) {
struct rps_dev_flow voidflow, *rflow = &voidflow;
int cpu;
preempt_disable();
rcu_read_lock();
cpu = get_rps_cpu(skb->dev, skb, &rflow);
if (cpu < 0)
cpu = smp_processor_id();
ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
rcu_read_unlock();
preempt_enable();
} else
#endif
This code needs the preempt_disable().
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> net/core/dev.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1baab07820f65..325b70074f4ae 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4796,7 +4796,6 @@ static int netif_rx_internal(struct sk_buff *skb)
> struct rps_dev_flow voidflow, *rflow = &voidflow;
> int cpu;
>
> - preempt_disable();
> rcu_read_lock();
>
> cpu = get_rps_cpu(skb->dev, skb, &rflow);
> @@ -4806,7 +4805,6 @@ static int netif_rx_internal(struct sk_buff *skb)
> ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
>
> rcu_read_unlock();
> - preempt_enable();
> } else
> #endif
> {
> --
> 2.34.1
>
