Discuss why we need to unlink file path before closes fd.
Signed-off-by: Wei Liu <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Ian Jackson <[email protected]>
---
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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