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

From: Jürgen Groß
Date: Mon Feb 08 2021 - 05:54:04 EST

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@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);
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

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: application/pgp-keys

Attachment: OpenPGP_signature
Description: OpenPGP digital signature