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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        netdev@vger.kernel.org
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>, sassmann@redhat.com,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, stable-rt@vger.kernel.org
Subject: Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT
In-Reply-To: <20210512214324.hiaiw3e2tzmsygcz@linutronix.de>
References: <YJofplWBz8dT7xiw@localhost.localdomain> <20210512214324.hiaiw3e2tzmsygcz@linutronix.de>
Date:   Thu, 13 May 2021 00:28:02 +0200
Message-ID: <87k0o360zx.ffs@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, May 12 2021 at 23:43, Sebastian Andrzej Siewior wrote:
> __napi_schedule_irqoff() is an optimized version of __napi_schedule()
> which can be used where it is known that interrupts are disabled,
> e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer
> callbacks.
>
> On PREEMPT_RT enabled kernels this assumptions is not true. Force-
> threaded interrupt handlers and spinlocks are not disabling interrupts
> and the NAPI hrtimer callback is forced into softirq context which runs
> with interrupts enabled as well.
>
> Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole
> game so make __napi_schedule_irqoff() invoke __napi_schedule() for
> PREEMPT_RT kernels.
>
> The callers of ____napi_schedule() in the networking core have been
> audited and are correct on PREEMPT_RT kernels as well.
>
> Reported-by: Juri Lelli <juri.lelli@redhat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

> ---
> Alternatively __napi_schedule_irqoff() could be #ifdef'ed out on RT and
> an inline provided which invokes __napi_schedule().
>
> This was not chosen as it creates #ifdeffery all over the place and with
> the proposed solution the code reflects the documentation consistently
> and in one obvious place.

Blame me for that decision.

No matter which variant we end up with, this needs to go into all stable
RT kernels ASAP.

Thanks,

        tglx