libbb: make rtc_xopen try harder on EBUSY

function                                             old     new   delta
rtc_xopen                                             77     139     +62

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/libbb/rtc.c b/libbb/rtc.c
index 97455e86ae187a359e7dbc4e94f846406bb19cfd..f84da09bb2a92e4bb83bbdbb08d7e610c5a60045 100644 (file)
--- a/libbb/rtc.c
+++ b/libbb/rtc.c
@@ -33,23 +33,55 @@ int FAST_FUNC rtc_adjtime_is_utc(void)
        return utc;
 }
 
+/* rtc opens are exclusive.
+ * Try to run two "hwclock -w" at the same time to see it.
+ * Users wouldn't expect that to fail merely because /dev/rtc
+ * was momentarily busy, let's try a bit harder on errno == EBUSY.
+ */
+static int open_loop_on_busy(const char *name, int flags)
+{
+       int rtc;
+       /*
+        * Tested with two parallel "hwclock -w" loops.
+        * With try = 10, no failures with 2x1000000 loop iterations.
+        */
+       int try = 1000 / 20;
+ again:
+       errno = 0;
+       rtc = open(name, flags);
+       if (errno == EBUSY) {
+               usleep(20 * 1000);
+               if (--try != 0)
+                       goto again;
+               /* EBUSY. Last try, exit on error instead of returning -1 */
+               return xopen(name, flags);
+       }
+       return rtc;
+}
+
+/* Never fails */
 int FAST_FUNC rtc_xopen(const char **default_rtc, int flags)
 {
        int rtc;
+       const char *name =
+               "/dev/rtc""\0"
+               "/dev/rtc0""\0"
+               "/dev/misc/rtc""\0";
 
-       if (!*default_rtc) {
-               *default_rtc = "/dev/rtc";
-               rtc = open(*default_rtc, flags);
-               if (rtc >= 0)
-                       return rtc;
-               *default_rtc = "/dev/rtc0";
-               rtc = open(*default_rtc, flags);
+       if (!*default_rtc)
+               goto try_name;
+       name = ""; /*else: we have rtc name, don't try other names */
+
+       for (;;) {
+               rtc = open_loop_on_busy(*default_rtc, flags);
                if (rtc >= 0)
                        return rtc;
-               *default_rtc = "/dev/misc/rtc";
+               name += strlen(name) + 1;
+               if (!name[0])
+                       return xopen(*default_rtc, flags);
+ try_name:
+               *default_rtc = name;
        }
-
-       return xopen(*default_rtc, flags);
 }
 
 void FAST_FUNC rtc_read_tm(struct tm *ptm, int fd)