[PATCH 01 of 29 RFC] libxl: Atomicaly check backend state and set it to 5 at device_remove

[Roger Pau Monne <roger.pau@entel.upc.edu>]

# HG changeset patch
# User Roger Pau Monne <roger.pau@entel.upc.edu>
# Date 1326548456 -3600
# Node ID e38c059da393c3fa2ee222d2e90275ca88b85d4f
# Parent  e2722b24dc0962de37215320b05d1bb7c4c42864
libxl: Atomicaly check backend state and set it to 5 at device_remove

libxl__device_remove was setting backend state to 5, which could
create a race condition, since the previous check for state != 4 and
setting state to 5 was not done inside the same transaction, so the
kernel could change the state to 6 in the space between the check for
state != 4 and setting it to 5.

The state != 4 check and setting it to 5 should happen in the same
transaction, to assure that nobody is modifying it behind our back.

Changes since v1:

 * Do the check and set in the same transaction, instead of removing
   the set state to 5.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>

diff -r e2722b24dc09 -r e38c059da393 tools/libxl/libxl_device.c
--- a/tools/libxl/libxl_device.c	Thu Jan 26 17:43:31 2012 +0000
+++ b/tools/libxl/libxl_device.c	Sat Jan 14 14:40:56 2012 +0100
@@ -433,19 +433,23 @@ int libxl__device_remove(libxl__gc *gc, 
     xs_transaction_t t;
     char *be_path = libxl__device_backend_path(gc, dev);
     char *state_path = libxl__sprintf(gc, "%s/state", be_path);
-    char *state = libxl__xs_read(gc, XBT_NULL, state_path);
+    char *state;
     int rc = 0;
 
-    if (!state)
+retry_transaction:
+    t = xs_transaction_start(ctx->xsh);
+    state = libxl__xs_read(gc, t, state_path);
+    if (!state) {
+        xs_transaction_end(ctx->xsh, t, 0);
         goto out;
+    }
     if (atoi(state) != 4) {
+        xs_transaction_end(ctx->xsh, t, 0);
         libxl__device_destroy_tapdisk(gc, be_path);
         xs_rm(ctx->xsh, XBT_NULL, be_path);
         goto out;
     }
 
-retry_transaction:
-    t = xs_transaction_start(ctx->xsh);
     xs_write(ctx->xsh, t, libxl__sprintf(gc, "%s/online", be_path), "0", strlen("0"));
     xs_write(ctx->xsh, t, state_path, "5", strlen("5"));
     if (!xs_transaction_end(ctx->xsh, t, 0)) {