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Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
To:     Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-8-jgross@suse.com>
 <8032d8a9-b28f-95e1-a5a8-e955ada4dc0a@xen.org>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <969f1492-764b-3345-eb65-64aca554ce9e@suse.com>
Date:   Mon, 8 Feb 2021 12:48:49 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
 linux-kernel@vger.kernel.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>
Message-ID: <969f1492-764b-3345-eb65-64aca554ce9e@suse.com>
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-8-jgross@suse.com>
 <8032d8a9-b28f-95e1-a5a8-e955ada4dc0a@xen.org>
In-Reply-To: <8032d8a9-b28f-95e1-a5a8-e955ada4dc0a@xen.org>

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On 08.02.21 12:40, Julien Grall wrote:
>=20
>=20
> On 06/02/2021 10:49, Juergen Gross wrote:
>> In evtchn_read() use READ_ONCE() for reading the producer index in
>> order to avoid the compiler generating multiple accesses.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> =C2=A0 drivers/xen/evtchn.c | 2 +-
>> =C2=A0 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
>> index 421382c73d88..f6b199b597bf 100644
>> --- a/drivers/xen/evtchn.c
>> +++ b/drivers/xen/evtchn.c
>> @@ -211,7 +211,7 @@ static ssize_t evtchn_read(struct file *file, char=
=20
>> __user *buf,
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 goto unlock_out;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 c =3D u->ring_c=
ons;
>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p =3D u->ring_prod;
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p =3D READ_ONCE(u->ring_pr=
od);
> For consistency, don't you also need the write side in=20
> evtchn_interrupt() to use WRITE_ONCE()?

Only in case I'd consider the compiler needing multiple memory
accesses for doing the update (see my reply to Jan's comment on this
patch).


Juergen

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