From: Halil Pasic <pasic@linux.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>,
Matthew Rosato <mjrosato@linux.ibm.com>,
David Hildenbrand <david@redhat.com>,
Cornelia Huck <cohuck@redhat.com>,
qemu-devel@nongnu.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
qemu-s390x@nongnu.org, Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic
Date: Mon, 6 Jul 2020 13:19:09 +0200 [thread overview]
Message-ID: <20200706131909.6c6ad79b.pasic@linux.ibm.com> (raw)
In-Reply-To: <20200704143126-mutt-send-email-mst@kernel.org>
On Sat, 4 Jul 2020 14:34:04 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Tue, Jun 16, 2020 at 06:50:34AM +0200, Halil Pasic wrote:
> > The atomic_cmpxchg() loop is broken because we occasionally end up with
> > old and _old having different values (a legit compiler can generate code
> > that accessed *ind_addr again to pick up a value for _old instead of
> > using the value of old that was already fetched according to the
> > rules of the abstract machine). This means the underlying CS instruction
> > may use a different old (_old) than the one we intended to use if
> > atomic_cmpxchg() performed the xchg part.
>
> And was this ever observed in the field? Or is this a theoretical issue?
> commit log should probably say ...
>
It was observed in the field (Christian already answered). I think the
message already implies this, because the only conjunctive is about the
compiler behavior.
> >
> > Let us use volatile to force the rules of the abstract machine for
> > accesses to *ind_addr. Let us also rewrite the loop so, we that the
>
> we that -> we know that?
s/we//
It would be nice to fix this before the patch gets merged.
>
> > new old is used to compute the new desired value if the xchg part
> > is not performed.
> >
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > Reported-by: Andre Wild <Andre.Wild1@ibm.com>
> > Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
> > ---
> > hw/s390x/virtio-ccw.c | 18 ++++++++++--------
> > 1 file changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index c1f4bb1d33..3c988a000b 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -786,9 +786,10 @@ static inline VirtioCcwDevice *to_virtio_ccw_dev_fast(DeviceState *d)
> > static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> > uint8_t to_be_set)
> > {
> > - uint8_t ind_old, ind_new;
> > + uint8_t expected, actual;
> > hwaddr len = 1;
> > - uint8_t *ind_addr;
> > + /* avoid multiple fetches */
> > + uint8_t volatile *ind_addr;
> >
> > ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
> > if (!ind_addr) {
> > @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> > __func__, sch->cssid, sch->ssid, sch->schid);
> > return -1;
> > }
> > + actual = *ind_addr;
> > do {
> > - ind_old = *ind_addr;
> > - ind_new = ind_old | to_be_set;
> > - } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old);
> > - trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new);
> > - cpu_physical_memory_unmap(ind_addr, len, 1, len);
> > + expected = actual;
> > + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
> > + } while (actual != expected);
> > + trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set);
> > + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
> >
> > - return ind_old;
> > + return actual;
> > }
>
> I wonder whether cpuXX APIs should accept volatile pointers, too:
> casting away volatile is always suspicious.
> But that is a separate issue ...
>
Nod.
Thanks for having a look!
> > static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
> > --
> > 2.17.1
>
next prev parent reply other threads:[~2020-07-06 11:22 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-16 4:50 [PATCH 0/2] two atomic_cmpxchg() related fixes Halil Pasic
2020-06-16 4:50 ` [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic Halil Pasic
2020-06-16 5:58 ` Christian Borntraeger
2020-06-16 6:33 ` Cornelia Huck
2020-06-16 6:45 ` Christian Borntraeger
2020-06-19 7:14 ` Cornelia Huck
2020-06-17 23:56 ` Halil Pasic
2020-06-19 7:33 ` David Hildenbrand
2020-06-19 8:17 ` Halil Pasic
2020-06-16 9:31 ` Halil Pasic
2020-07-01 13:13 ` Christian Borntraeger
2020-07-04 18:34 ` Michael S. Tsirkin
2020-07-06 5:44 ` Christian Borntraeger
2020-07-06 11:19 ` Halil Pasic [this message]
2020-06-16 4:50 ` [PATCH 2/2] s390x/pci: fix set_ind_atomic Halil Pasic
2020-07-01 13:14 ` Christian Borntraeger
2020-07-01 12:01 ` [PATCH 0/2] two atomic_cmpxchg() related fixes Cornelia Huck
2020-07-01 12:06 ` Christian Borntraeger
2020-07-01 13:10 ` Cornelia Huck
2020-07-03 13:37 ` Halil Pasic
2020-07-03 14:03 ` Cornelia Huck
2020-07-02 11:18 ` Cornelia Huck
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