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From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Halil Pasic <pasic@linux.ibm.com>,
	Matthew Rosato <mjrosato@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	qemu-s390x@nongnu.org, qemu-devel@nongnu.org,
	Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>,
	Richard Henderson <rth@twiddle.net>,
	Andreas Krebbel <krebbel@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic
Date: Tue, 16 Jun 2020 07:58:53 +0200
Message-ID: <11e8278e-23cc-1e7f-4086-10ecef75b96a@de.ibm.com> (raw)
In-Reply-To: <20200616045035.51641-2-pasic@linux.ibm.com>



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;                                                               \   
 
 })
 
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. 




> -    } 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)
> 




  reply index

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 [this message]
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
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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