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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@xxxxxxxx>
  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

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.



Julien Grall



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