Re: [Xen-devel] [PATCH v2 3/4] xen: add process_pending_softirqs_norcu() for keyhandlers

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Juergen Gross <jgross@suse.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>,
	Jun Nakajima <jun.nakajima@intel.com>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH v2 3/4] xen: add process_pending_softirqs_norcu() for keyhandlers
Date: Mon, 24 Feb 2020 12:25:39 +0100	[thread overview]
Message-ID: <20200224112539.GL4679@Air-de-Roger> (raw)
In-Reply-To: <20200218122114.17596-4-jgross@suse.com>

On Tue, Feb 18, 2020 at 01:21:13PM +0100, Juergen Gross wrote:
> Some keyhandlers are calling process_pending_softirqs() while holding
> a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
> activate rcu calls which should not happen inside a rcu_read_lock().

It might be helpful to turn the ASSERT in process_pending_softirqs
into ASSERT_NOT_IN_ATOMIC also, as it would catch such missuses
AFAICT.

> 
> For that purpose add process_pending_softirqs_norcu() which will not
> do any rcu activity and use this for keyhandlers.

I wonder if for keyhandlers it might be easier to just disable the
watchdog in handle_keypress and remove the softirq processing from the
handlers.

At the end of day we want the keyhanders to run as fast as possible in
order to get the data out, and we only care about the watchdog not
triggering? (maybe I'm missing something here)

> +void process_pending_softirqs_norcu(void)
> +{
> +    ASSERT(!in_irq() && local_irq_is_enabled());
> +    /* Do not enter scheduler as it can preempt the calling context. */
> +    __do_softirq((1ul << SCHEDULE_SOFTIRQ) | (1ul << SCHED_SLAVE_SOFTIRQ),

Don't you also need to pass RCU_SOFTIRQ to the ignore mask in order to
avoid any RCU work happening?

Thanks, Roger.

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