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

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Date:   Tue, 10 Mar 2020 02:30:34 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "brouer@...hat.com" <brouer@...hat.com>
Subject: Re: [PATCH] page_pool: use irqsave/irqrestore to protect ring access.

On Mon, 2020-03-09 at 17:55 -0700, David Miller wrote:
> 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.

a printk inside irq context lead to this, so maybe it can be avoided ..

or instead of checking in_serving_softirq()  change page_pool to
check in_interrupt() which is more powerful, to avoid ptr_ring locking
and the complication with netpoll altogether.

I wonder why Jesper picked in_serving_softirq() in first place, was
there a specific reason ? or he just wanted it to be as less strict as
possible ?