[Xen-devel] [PATCH 2/2] libxl_internal: comment on domain userdata unlock function

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Discuss why we need to unlink file path before closes fd.

Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 tools/libxl/libxl_internal.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
index 9d8025d..a70214b 100644
--- a/tools/libxl/libxl_internal.c
+++ b/tools/libxl/libxl_internal.c
@@ -458,6 +458,20 @@ out:
 
 void libxl__unlock_domain_userdata(libxl__domain_userdata_lock *lock)
 {
+    /* It's important to unlink the file before closing fd to avoid
+     * such race (if close before unlink):
+     *
+     *   P1 LOCK                         P2 UNLOCK
+     *   fd1 = open(lockfile)
+     *                                   close(fd2)
+     *   flock(fd1)
+     *   fstat and stat check success
+     *                                   unlink(lockfile)
+     *   return lock
+     *
+     * In above case P1 thinks it has got hold of the lock but
+     * actually lock is released by P2 (lockfile unlinked).
+     */
     if (lock->path) unlink(lock->path);
     if (lock->carefd) libxl__carefd_close(lock->carefd);
     free(lock->path);
-- 
1.7.10.4


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