Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

From: Julien Grall <julien@xen.org>
To: "Jürgen Groß" <jgross@suse.com>,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org, netdev@vger.kernel.org,
	linux-scsi@vger.kernel.org
Cc: "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	stable@vger.kernel.org,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	"Jens Axboe" <axboe@kernel.dk>, "Wei Liu" <wei.liu@kernel.org>,
	"Paul Durrant" <paul@xen.org>,
	"David S. Miller" <davem@davemloft.net>,
	"Jakub Kicinski" <kuba@kernel.org>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
Date: Mon, 8 Feb 2021 10:40:44 +0000	[thread overview]
Message-ID: <28399fd1-9fe8-f31a-6ee8-e78de567155b@xen.org> (raw)
In-Reply-To: <0d623c98-a714-1639-cc53-f58ba3f08212@suse.com>

Hi Juergen,

On 08/02/2021 10:22, Jürgen Groß wrote:
> On 08.02.21 10:54, Julien Grall wrote:
>> ... I don't really see how the difference matter here. The idea is to 
>> re-use what's already existing rather than trying to re-invent the 
>> wheel with an extra lock (or whatever we can come up).
> 
> The difference is that the race is occurring _before_ any IRQ is
> involved. So I don't see how modification of IRQ handling would help.

Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:

if ( irq in progress )
{
   set IRQS_PENDING
   return;
}

do
{
   clear IRQS_PENDING
   handle_irq()
} while (IRQS_PENDING is set)

IRQ handling flow like handle_fasteoi_irq() looks like:

if ( irq in progress )
   return;

handle_irq()

The latter flow would catch "spurious" interrupt and ignore them. So it 
would handle nicely the race when changing the event affinity.

Cheers,

-- 
Julien Grall