linux-kernel - Re: [PATCH 7/7] xen/evtchn: read producer index only once

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Message-ID: <a3f47092-0525-4594-0421-48e83cee5045@xen.org>
Date:   Mon, 8 Feb 2021 12:03:52 +0000
From:   Julien Grall <julien@....org>
To:     Jürgen Groß <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once

Hi Juergen,

On 08/02/2021 11:48, Jürgen Groß wrote:
> On 08.02.21 12:40, Julien Grall wrote:
>>
>>
>> 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@...e.com>
>>> ---
>>>   drivers/xen/evtchn.c | 2 +-
>>>   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 __user *buf,
>>>               goto unlock_out;
>>>           c = u->ring_cons;
>>> -        p = u->ring_prod;
>>> +        p = READ_ONCE(u->ring_prod);
>> For consistency, don't you also need the write side in 
>> 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).

Right, I have just answered there :). AFAICT, without using 
WRITE_ONCE()/READ_ONCE() there is no guarantee that load/store tearing 
will not happen.

We can continue the conversation there.

Cheers,

> 
> Juergen

-- 
Julien Grall