[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH 7/7] xen/evtchn: read producer index only once
On 08.02.21 13:23, Jan Beulich wrote:
On 08.02.2021 13:15, Jürgen Groß wrote:
On 08.02.21 12:54, Jan Beulich wrote:
On 08.02.2021 11:59, Jürgen Groß wrote:
On 08.02.21 11:51, Jan Beulich wrote:
On 08.02.2021 11:41, Jürgen Groß wrote:
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.
I still wouldn't understand the change (and the lack of
further changes) then: The first further use of p is
outside the loop, alongside one of c. IOW why would c
then not need treating the same as p?
Its value wouldn't change, as ring_cons is being modified only at
the bottom of this function, and nowhere else (apart from the reset
case, but this can't run concurrently due to ring_cons_mutex).
I also still don't see the difference between latching a
value into a local variable vs a "freestanding" access -
neither are guaranteed to result in exactly one memory
access afaict.
READ_ONCE() is using a pointer to volatile, so any refetching by
the compiler would be a bug.
Of course, but this wasn't my point. I was contrasting
c = u->ring_cons;
p = u->ring_prod;
which you change with
rc = wait_event_interruptible(u->evtchn_wait,
u->ring_cons != u->ring_prod);
which you leave alone.
Can you point out which problem might arise from that?
Not any particular active one. Yet enhancing some accesses
but not others seems to me like a recipe for new problems
down the road.
I already reasoned that the usage of READ_ONCE() is due to storing the
value in a local variable which needs to be kept constant during the
following processing (no refetches by the compiler). This reasoning
very clearly doesn't apply to the other accesses.
Juergen
Attachment:
OpenPGP_0xB0DE9DD628BF132F.asc
Description: application/pgp-keys
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature
|
