netdev - Re: [PATCH net v2] net: core: make napi

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Message-ID: <20210414162109.77eecf47@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 14 Apr 2021 16:21:09 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lijun Pan <lijunp213@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net v2] net: core: make napi_disable more robust

On Wed, 14 Apr 2021 03:08:45 -0500 Lijun Pan wrote:
> There are chances that napi_disable can be called twice by NIC driver.
> This could generate deadlock. For example,
> the first napi_disable will spin until NAPI_STATE_SCHED is cleared
> by napi_complete_done, then set it again.
> When napi_disable is called the second time, it will loop infinitely
> because no dev->poll will be running to clear NAPI_STATE_SCHED.
> 
> Though it is driver writer's responsibility to make sure it being
> called only once, making napi_disable more robust does not hurt, not
> to say it can prevent a buggy driver from crashing a system.
> So, we check the napi state bit to make sure that if napi is already
> disabled, we exit the call early enough to avoid spinning infinitely.

You've already been told by Eric & Dave to fix the driver instead.

Your check is _not_ correct - SCHED && NPSVC && !MISSED && !BUSY_POLL 
can well arise without disabling the NAPI.

But regardless, a driver bug should be relatively easy to identify with
task getting stuck in napi_disable(). We don't provide "protection" 
for taking spin locks or ref counts twice either. Unless you can show 
a strong use case please stop posting new versions of this patch.