commit d3faa13e5ffda779dcd58f39e8745370adb05d67
author Paolo Bonzini <pbonzini@redhat.com> Mon Jun 05 14:38:50 2017 +0200
committer Fam Zheng <famz@redhat.com> Fri Jun 16 07:55:00 2017 +0800
tree adc8d74261b334cba0a0bc52e30466af3c4694eb
parent 79f24568e5e70892b9bf4712f88b26f52255077f

block: access copy_on_read with atomic ops

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170605123908.18777-2-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

block.c

diff --git a/block.c b/block.c
index fa1d06d..af6366b 100644
--- a/block.c
+++ b/block.c
@@ -1300,7 +1300,9 @@
         goto fail_opts;
     }
 
-    assert(bs->copy_on_read == 0); /* bdrv_new() and bdrv_close() make it so */
+    /* bdrv_new() and bdrv_close() make it so */
+    assert(atomic_read(&bs->copy_on_read) == 0);
+
     if (bs->open_flags & BDRV_O_COPY_ON_READ) {
         if (!bs->read_only) {
             bdrv_enable_copy_on_read(bs);
@@ -3063,7 +3065,7 @@
 
         g_free(bs->opaque);
         bs->opaque = NULL;
-        bs->copy_on_read = 0;
+        atomic_set(&bs->copy_on_read, 0);
         bs->backing_file[0] = '\0';
         bs->backing_format[0] = '\0';
         bs->total_sectors = 0;

block/io.c

diff --git a/block/io.c b/block/io.c
index ed31810..98c690f 100644
--- a/block/io.c
+++ b/block/io.c
@@ -130,13 +130,13 @@
  */
 void bdrv_enable_copy_on_read(BlockDriverState *bs)
 {
-    bs->copy_on_read++;
+    atomic_inc(&bs->copy_on_read);
 }
 
 void bdrv_disable_copy_on_read(BlockDriverState *bs)
 {
-    assert(bs->copy_on_read > 0);
-    bs->copy_on_read--;
+    int old = atomic_fetch_dec(&bs->copy_on_read);
+    assert(old >= 1);
 }
 
 /* Check if any requests are in-flight (including throttled requests) */
@@ -1144,7 +1144,7 @@
     bdrv_inc_in_flight(bs);
 
     /* Don't do copy-on-read if we read data before write operation */
-    if (bs->copy_on_read && !(flags & BDRV_REQ_NO_SERIALISING)) {
+    if (atomic_read(&bs->copy_on_read) && !(flags & BDRV_REQ_NO_SERIALISING)) {
         flags |= BDRV_REQ_COPY_ON_READ;
     }
 

blockdev.c

diff --git a/blockdev.c b/blockdev.c
index 6472548..ceea976 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1791,7 +1791,7 @@
     /* We don't need (or want) to use the transactional
      * bdrv_reopen_multiple() across all the entries at once, because we
      * don't want to abort all of them if one of them fails the reopen */
-    if (!state->old_bs->copy_on_read) {
+    if (!atomic_read(&state->old_bs->copy_on_read)) {
         bdrv_reopen(state->old_bs, state->old_bs->open_flags & ~BDRV_O_RDWR,
                     NULL);
     }

include/block/block_int.h

diff --git a/include/block/block_int.h b/include/block/block_int.h
index cb78c4f..49f2ebb 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -595,11 +595,6 @@
 
     /* Protected by AioContext lock */
 
-    /* If true, copy read backing sectors into image.  Can be >1 if more
-     * than one client has requested copy-on-read.
-     */
-    int copy_on_read;
-
     /* If we are reading a disk image, give its size in sectors.
      * Generally read-only; it is written to by load_snapshot and
      * save_snaphost, but the block layer is quiescent during those.
@@ -633,6 +628,12 @@
 
     QLIST_HEAD(, BdrvDirtyBitmap) dirty_bitmaps;
 
+    /* If true, copy read backing sectors into image.  Can be >1 if more
+     * than one client has requested copy-on-read.  Accessed with atomic
+     * ops.
+     */
+    int copy_on_read;
+
     /* do we need to tell the quest if we have a volatile write cache? */
     int enable_write_cache;