linux-kernel - Re: [PATCH 1/2] net: Assert proper context while calling napi

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Message-ID: <20250214140045.547f1396@kernel.org>
Date: Fri, 14 Feb 2025 14:00:45 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Breno Leitao <leitao@...ian.org>, LKML <linux-kernel@...r.kernel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will
 Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Waiman Long
 <longman@...hat.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
 <horms@...nel.org>, Hayes Wang <hayeswang@...ltek.com>,
 linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: Assert proper context while calling
 napi_schedule()

On Thu, 13 Feb 2025 21:38:12 +0100 Frederic Weisbecker wrote:
> > > Just to make sure I follow the netpoll issue. What would you like to fix
> > > in netpoll exactly?  
> > 
> > Nothing in netpoll, the problem is that netdevsim calls napi_schedule
> > from the xmit path. That's incompatible with netpoll. We should fix
> > netdevsim instead (unless more real drivers need napi-from-xmit to
> > work).  
> 
> Let me clarify, because I don't know much this area. If the problem is that xmit
> can't call napi_schedule() by design, then I defer to you. But if the problem is that
> napi_schedule() may or may not be called from an interrupt, please note that
> local_bh_enable() won't run softirqs from a hardirq and will instead defer to
> IRQ tail. So it's fine to do an unconditional pair of local_bh_disable() / local_bh_enable().

I don't know where this is in the code TBH, but my understanding is
that HW IRQs - yes, as you say it'd be safe; the problem is that 
we have local_irq_save() all over the place. And that is neither
protected from local_bh_enable(), not does irq_restore execute softirqs.