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>

1 files changed, 42 insertions, 10 deletions

diff --git a/libbb/rtc.c b/libbb/rtc.c
index 97455e86a..f84da09bb 100644
--- 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)