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

[Jürgen Groß]

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 <jgr...@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;

-               p = u->ring_prod;
+               p = READ_ONCE(u->ring_prod);
                if (c != p)
                        break;

c = u->ring_cons;

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.

And of course there's also our beloved topic of access
tearing here: READ_ONCE() also excludes that (at least as
per its intentions aiui).

Yes, but I don't see an urgent need to fix that, as there would
be thousands of accesses in the kernel needing a fix. A compiler
tearing a naturally aligned access into multiple memory accesses
would be rejected as buggy from the kernel community IMO.


Juergen

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