[PATCH v7 24/38] struct lock_file: declare some fields volatile

From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Oct 1, 2014, 10:28 UTC
Message-ID: <1412159322-2622-25-git-send-email-mhagger@alum.mit.edu>
In-Reply-To: <1412159322-2622-1-git-send-email-mhagger@alum.mit.edu>

The function remove_lock_file_on_signal() is used as a signal handler. It is not realistic to make the signal handler conform strictly to the C standard, which is very restrictive about what a signal handler is allowed to do. But let's increase the likelihood that it will work:

The lock_file_list global variable and several fields from struct lock_file are used by the signal handler. Declare those values "volatile" to (1) force the main process to write the values to RAM promptly, and (2) prevent updates to these fields from being reordered in a way that leaves an opportunity for a jump to the signal handler while the object is in an inconsistent state.

We don't mark the filename field volatile because that would prevent the use of strcpy(), and it is anyway unlikely that a compiler re-orders a strcpy() call across other expressions. So in practice it should be possible to get away without "volatile" in the "filename" case.

Suggested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 cache.h    | 6 +++---
 lockfile.c | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/cache.h b/cache.h
index 8e25fce..c2ea6f1 100644
--- a/cache.h
+++ b/cache.h
@@ -575,10 +575,10 @@ extern int refresh_index(struct index_state *, unsigned int flags, const struct
 #define LOCK_SUFFIX_LEN 5
 
 struct lock_file {
-	struct lock_file *next;
+	struct lock_file *volatile next;
 	volatile sig_atomic_t active;
-	int fd;
-	pid_t owner;
+	volatile int fd;
+	volatile pid_t owner;
 	char on_list;
 	char filename[PATH_MAX];
 };
diff --git a/lockfile.c b/lockfile.c
index d35ac44..89043f5 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -55,7 +55,7 @@
  *     on_list is set.
  */
 
-static struct lock_file *lock_file_list;
+static struct lock_file *volatile lock_file_list;
 
 static void remove_lock_file(void)
 {

-- 
2.1.0