[Xen-changelog] [xen-unstable] tools/libvchan: Beef up the CPU barriers in libvchan.

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir@xxxxxxx>
# Date 1327166140 0
# Node ID 80fdf2182bc62ca358ba2f1a3513b47a4f8d9dfd
# Parent  475d73479663b7bf85da84a7794df762aa2289a3
tools/libvchan: Beef up the CPU barriers in libvchan.

Although they were sufficient for x86, they weren't safe more generally.

Signed-off-by: Keir Fraser <keir@xxxxxxx>
---


diff -r 475d73479663 -r 80fdf2182bc6 tools/libvchan/io.c
--- a/tools/libvchan/io.c       Sat Jan 21 17:14:37 2012 +0000
+++ b/tools/libvchan/io.c       Sat Jan 21 17:15:40 2012 +0000
@@ -55,9 +55,6 @@
 #define VCHAN_DEBUG 0
 #endif
 
-#define barrier() asm volatile("" ::: "memory")
-
-
 static inline uint32_t rd_prod(struct libxenvchan *ctrl)
 {
        return ctrl->read.shr->prod;
@@ -104,12 +101,15 @@
 {
        uint8_t *notify = ctrl->is_server ? &ctrl->ring->cli_notify : 
&ctrl->ring->srv_notify;
        __sync_or_and_fetch(notify, bit);
+       xen_mb(); /* post the request /before/ caller re-reads any indexes */
 }
 
 static inline int send_notify(struct libxenvchan *ctrl, uint8_t bit)
 {
-       uint8_t *notify = ctrl->is_server ? &ctrl->ring->srv_notify : 
&ctrl->ring->cli_notify;
-       uint8_t prev = __sync_fetch_and_and(notify, ~bit);
+       uint8_t *notify, prev;
+       xen_mb(); /* caller updates indexes /before/ we decode to notify */
+       notify = ctrl->is_server ? &ctrl->ring->srv_notify : 
&ctrl->ring->cli_notify;
+       prev = __sync_fetch_and_and(notify, ~bit);
        if (prev & bit)
                return xc_evtchn_notify(ctrl->event, ctrl->event_port);
        else
@@ -197,15 +197,15 @@
        }
        if (avail_contig > size)
                avail_contig = size;
+       xen_mb(); /* read indexes /then/ write data */
        memcpy(wr_ring(ctrl) + real_idx, data, avail_contig);
        if (avail_contig < size)
        {
                // we rolled across the end of the ring
                memcpy(wr_ring(ctrl), data + avail_contig, size - avail_contig);
        }
-       barrier(); // data must be in the ring prior to increment
+       xen_wmb(); /* write data /then/ notify */
        wr_prod(ctrl) += size;
-       barrier(); // increment must happen prior to notify
        if (send_notify(ctrl, VCHAN_NOTIFY_WRITE))
                return -1;
        return size;
@@ -268,13 +268,14 @@
        int avail_contig = rd_ring_size(ctrl) - real_idx;
        if (avail_contig > size)
                avail_contig = size;
-       barrier(); // data read must happen after rd_cons read
+       xen_rmb(); /* data read must happen /after/ rd_cons read */
        memcpy(data, rd_ring(ctrl) + real_idx, avail_contig);
        if (avail_contig < size)
        {
                // we rolled across the end of the ring
                memcpy(data + avail_contig, rd_ring(ctrl), size - avail_contig);
        }
+       xen_mb(); /* consume /then/ notify */
        rd_cons(ctrl) += size;
        if (VCHAN_DEBUG) {
                char metainfo[32];
@@ -285,7 +286,6 @@
                iov[1].iov_len = size;
                writev(-1, iov, 2);
        }
-       barrier(); // consumption must happen prior to notify of newly freed 
space
        if (send_notify(ctrl, VCHAN_NOTIFY_READ))
                return -1;
        return size;

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