On Fri, 03/13 09:09, Paolo Bonzini wrote:
>
>
> On 13/03/2015 02:38, Fam Zheng wrote:
> > There could be a race condition when two processes call
> > address_space_map concurrently and both want to use the bounce buffer.
> >
> > Add an in_use flag in BounceBuffer to sync it.
> >
> > Signed-off-by: Fam Zheng <f...@redhat.com>
> > ---
> > exec.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/exec.c b/exec.c
> > index 60b9752..8d4e134 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -2481,6 +2481,7 @@ typedef struct {
> > void *buffer;
> > hwaddr addr;
> > hwaddr len;
> > + bool in_use;
> > } BounceBuffer;
> >
> > static BounceBuffer bounce;
> > @@ -2569,9 +2570,10 @@ void *address_space_map(AddressSpace *as,
> > l = len;
> > mr = address_space_translate(as, addr, &xlat, &l, is_write);
> > if (!memory_access_is_direct(mr, is_write)) {
> > - if (bounce.buffer) {
> > + if (atomic_cmpxchg(&bounce.in_use, false, true)) {
>
> atomic_or is enough...
atomic_cmpxchg is here to take the ownership of bounce iff it is not in use, so
I think it is necessary.
Fam
>
> > return NULL;
> > }
> > + smp_mb();
>
> ... and it already includes a memory barrier.
>
> Paolo
>
> > /* Avoid unbounded allocations */
> > l = MIN(l, TARGET_PAGE_SIZE);
> > bounce.buffer = qemu_memalign(TARGET_PAGE_SIZE, l);
> > @@ -2639,6 +2641,7 @@ void address_space_unmap(AddressSpace *as, void
> > *buffer, hwaddr len,
> > qemu_vfree(bounce.buffer);
> > bounce.buffer = NULL;
> > memory_region_unref(bounce.mr);
> > + atomic_mb_set(&bounce.in_use, false);
> > cpu_notify_map_clients();
> > }
> >
> >
