[Xen-changelog] [xen stable-4.6] libvchan: Read prod/cons only once.

[patchbot@xxxxxxx]

commit e0493703a96c03aaafc8d179624b89b7cf600916
Author:     Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
AuthorDate: Wed Mar 9 16:56:53 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Mar 9 16:56:53 2016 +0100

    libvchan: Read prod/cons only once.
    
    We must ensure that the prod/cons are only read once and that
    the compiler won't try to optimize the reads. That is split
    the read of these in multiple instructions influencing later
    branch code. As such insert barriers when fetching the cons
    and prod index.
    
    This is part of XSA155.
    
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
    master commit: 7d66a4ba695ab8d13b214fb816dd59e443ae1ec9
    master date: 2015-12-18 09:50:02 -0500
---
 tools/libvchan/io.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/libvchan/io.c b/tools/libvchan/io.c
index 8a9629b..381cc05 100644
--- a/tools/libvchan/io.c
+++ b/tools/libvchan/io.c
@@ -117,6 +117,7 @@ static inline int send_notify(struct libxenvchan *ctrl, 
uint8_t bit)
 static inline int raw_get_data_ready(struct libxenvchan *ctrl)
 {
        uint32_t ready = rd_prod(ctrl) - rd_cons(ctrl);
+       xen_mb(); /* Ensure 'ready' is read only once. */
        if (ready > rd_ring_size(ctrl))
                /* We have no way to return errors.  Locking up the ring is
                 * better than the alternatives. */
@@ -158,6 +159,7 @@ int libxenvchan_data_ready(struct libxenvchan *ctrl)
 static inline int raw_get_buffer_space(struct libxenvchan *ctrl)
 {
        uint32_t ready = wr_ring_size(ctrl) - (wr_prod(ctrl) - wr_cons(ctrl));
+       xen_mb(); /* Ensure 'ready' is read only once. */
        if (ready > wr_ring_size(ctrl))
                /* We have no way to return errors.  Locking up the ring is
                 * better than the alternatives. */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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