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

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Date:   Mon, 8 Feb 2021 12:48:49 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Julien Grall <julien@....org>, 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

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).


Juergen

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