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

Jürgen Groß
2021-02-08T10:54:00Z

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On 08.02.21 10:48, Jan Beulich wrote:
> On 06.02.2021 11:49, 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>
>> ---
>> 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);
>> if (c != p)
>> break;
>
> Why only here and not also in
>
> rc = wait_event_interruptible(u->evtchn_wait,
> u->ring_cons != u->ring_prod);
>
> or in evtchn_poll()? I understand it's not needed when
> ring_prod_lock is held, but that's not the case in the two
> afaics named places. Plus isn't the same then true for
> ring_cons and ring_cons_mutex, i.e. aren't the two named
> places plus evtchn_interrupt() also in need of READ_ONCE()
> for ring_cons?

The problem solved here is the further processing using "p" multiple
times. p must not be silently replaced with u->ring_prod by the
compiler, so I probably should reword the commit message to say:

... in order to not allow the compiler to refetch p.

Juergen

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