[PATCH] fix gettimeofday vs. update_gtod race
From: Nathan Lynch <hidden>
Date: 2006-08-11 20:41:17
Subsystem:
linux for powerpc (32-bit and 64-bit), the rest · Maintainers:
Madhavan Srinivasan, Linus Torvalds
Userspace can (very, very) occasionally get bogus time values due to a tiny race between powerpc's do_gettimeofday and timer interrupt: 1. do_gettimeofday does get_tb() 2. decrementer exception on boot cpu which runs timer_recalc_offset, which also samples the timebase and updates the do_gtod structure with a greater timebase value. 3. do_gettimeofday calls __do_gettimeofday, which leads to the negative result from tb_val - temp_varp->tb_orig_stamp. The fix is to ensure that do_gettimeofday samples the timebase only after loading do_gtod.varp. Signed-off-by: Nathan Lynch <redacted>
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 774c0a3..6b690df 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c@@ -417,7 +417,7 @@ static __inline__ void timer_check_rtc(v /* * This version of gettimeofday has microsecond resolution. */ -static inline void __do_gettimeofday(struct timeval *tv, u64 tb_val) +static inline void __do_gettimeofday(struct timeval *tv) { unsigned long sec, usec; u64 tb_ticks, xsec;
@@ -431,7 +431,12 @@ static inline void __do_gettimeofday(str * without a divide (and in fact, without a multiply) */ temp_varp = do_gtod.varp; - tb_ticks = tb_val - temp_varp->tb_orig_stamp; + + /* Sampling the time base must be done after loading + * do_gtod.varp in order to avoid racing with update_gtod. + */ + rmb(); + tb_ticks = get_tb() - temp_varp->tb_orig_stamp; temp_tb_to_xs = temp_varp->tb_to_xs; temp_stamp_xsec = temp_varp->stamp_xsec; xsec = temp_stamp_xsec + mulhdu(tb_ticks, temp_tb_to_xs);
@@ -464,7 +469,7 @@ void do_gettimeofday(struct timeval *tv) tv->tv_usec = usec; return; } - __do_gettimeofday(tv, get_tb()); + __do_gettimeofday(tv); } EXPORT_SYMBOL(do_gettimeofday);