netdev - Re: [PATCH] page_pool: use irqsave/irqrestore to protect ring access.

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Date:   Mon, 09 Mar 2020 17:55:34 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jonathan.lemon@...il.com
Cc:     netdev@...r.kernel.org, brouer@...hat.com,
        ilias.apalodimas@...aro.org, kernel-team@...com
Subject: Re: [PATCH] page_pool: use irqsave/irqrestore to protect ring
 access.

From: Jonathan Lemon <jonathan.lemon@...il.com>
Date: Mon, 9 Mar 2020 12:49:29 -0700

> netpoll may be called from IRQ context, which may access the
> page pool ring.  The current _bh variants do not provide sufficient
> protection, so use irqsave/restore instead.
> 
> Error observed on a modified mlx4 driver, but the code path exists
> for any driver which calls page_pool_recycle from napi poll.
> 
> WARNING: CPU: 34 PID: 550248 at /ro/source/kernel/softirq.c:161 __local_bh_enable_ip+0x35/0x50
 ...
> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>

The netpoll stuff always makes the locking more complicated than it needs
to be.  I wonder if there is another way around this issue?

Because IRQ save/restore is a high cost to pay in this critical path.