From: Halil Pasic <pasic@linux.ibm.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>,
Matthew Rosato <mjrosato@linux.ibm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Cornelia Huck <cohuck@redhat.com>,
David Hildenbrand <david@redhat.com>,
qemu-devel@nongnu.org, qemu-s390x@nongnu.org,
Paolo Bonzini <pbonzini@redhat.com>,
Andreas Krebbel <krebbel@linux.ibm.com>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic
Date: Tue, 16 Jun 2020 11:31:23 +0200 [thread overview]
Message-ID: <20200616113123.27d7d3f2.pasic@linux.ibm.com> (raw)
In-Reply-To: <11e8278e-23cc-1e7f-4086-10ecef75b96a@de.ibm.com>
On Tue, 16 Jun 2020 07:58:53 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>
>
> On 16.06.20 06:50, 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.
> >
> > 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
> > 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;
>
> to make things easier to understand. Adding a barrier in here also fixes the issue.
> Reasoning follows below:
>
> > - ind_new = ind_old | to_be_set;
>
> with an analysis from Andreas (cc)
>
> #define atomic_cmpxchg__nocheck(ptr, old, new) ({ \
>
> typeof_strip_qual(*ptr) _old = (old); \
>
> (void)__atomic_compare_exchange_n(ptr, &_old, new, false, \
>
> __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \
>
> _old; \
>
> })
>
There is also the
#define atomic_cmpxchg(ptr, old, new) __sync_val_compare_and_swap(ptr, old, new)
variant, I guess, when the C11 stuff is not available. I don't know if
that variant is guaranteed to not have problems with multiple loads.
> ind_old is copied into _old in the macro. Instead of doing the copy from the
> register the compiler reloads the value from memory. The result is that _old
> and ind_old end up having different values. _old in r1 with the bits set
> already and ind_old in r10 with the bits cleared. _old gets updated by CS
> and matches ind_old afterwards - both with the bits being 0. So the !=
> compare is false and the loop is left without having set any bits.
>
>
> Paolo (to),
> I am asking myself if it would be safer to add a barrier or something like
> this in the macros in include/qemu/atomic.h.
>
I think accessing the initial value via a volatile pointer initially and
using the value loaded by cmpxchg for subsequent iterations is cleaner.
Regards,
Halil
>
> > - } 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;
> > }
> >
> > static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
> >
>
>
>
next prev parent reply other threads:[~2020-06-16 9:33 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 [this message]
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
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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