Messages in this thread |  | | Subject | Re: [PATCH 7/7] xen/evtchn: read producer index only once | From | Julien Grall <> | Date | Mon, 8 Feb 2021 12:03:52 +0000 |
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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@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); >> 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
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