[PATCH 110/110] lib/vdso: Improve do_hres() and update vdso data unconditionally

[Huacai Chen]

In do_hres(), we currently use whether the return value of __arch_get_
hw_counter() is negative to indicate fallback, but this is not a good
idea because:

1, ARM64 returns ULL_MAX but MIPS returns 0 when clock_mode is invalid;
2, For a 64bit counter, a "negative" value of counter is actually valid.

It is sure that MIPS has a bug when clock_mode is invalid and should
return ULL_MAX as ARM64 does (Vincenzo has already submitted a patch).
But do_hres() can still be improved so we use U64_MAX as the only
"invalid" return value -- this is still not fully correct, but it is
the simplest fix and has no problem in most cases (we can hardly see a
64bit counter overflow).

By the way, currently update_vdso_data() and update_vsyscall_tz() rely
on __arch_use_vsyscall(), which causes __cvdso_clock_getres() and some
other functions get wrong results when clock_mode is invalid. So, we
update vdso data unconditionally.

Fixes: 00b26474c2f1613d7ab894c5 ("lib/vdso: Provide generic VDSO 
implementation")
Fixes: 44f57d788e7deecb50484353 ("timekeeping: Provide a generic 
update_vsyscall() implementation")
Cc: sta...@vger.kernel.org
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Paul Burton <paul.bur...@mips.com>
Cc: linux-m...@vger.kernel.org
Cc: linux-arm-ker...@lists.infradead.org
Signed-off-by: Huacai Chen <che...@lemote.com>
---
 kernel/time/vsyscall.c  | 9 +++------
 lib/vdso/gettimeofday.c | 2 +-
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
index 4bc37ac..5ee0f77 100644
--- a/kernel/time/vsyscall.c
+++ b/kernel/time/vsyscall.c
@@ -110,8 +110,7 @@ void update_vsyscall(struct timekeeper *tk)
        nsec            = nsec + tk->wall_to_monotonic.tv_nsec;
        vdso_ts->sec    += __iter_div_u64_rem(nsec, NSEC_PER_SEC, 
&vdso_ts->nsec);
 
-       if (__arch_use_vsyscall(vdata))
-               update_vdso_data(vdata, tk);
+       update_vdso_data(vdata, tk);
 
        __arch_update_vsyscall(vdata, tk);
 
@@ -124,10 +123,8 @@ void update_vsyscall_tz(void)
 {
        struct vdso_data *vdata = __arch_get_k_vdso_data();
 
-       if (__arch_use_vsyscall(vdata)) {
-               vdata[CS_HRES_COARSE].tz_minuteswest = sys_tz.tz_minuteswest;
-               vdata[CS_HRES_COARSE].tz_dsttime = sys_tz.tz_dsttime;
-       }
+       vdata[CS_HRES_COARSE].tz_minuteswest = sys_tz.tz_minuteswest;
+       vdata[CS_HRES_COARSE].tz_dsttime = sys_tz.tz_dsttime;
 
        __arch_sync_vdso_data(vdata);
 }
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index e630e7f..5a31643 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -50,7 +50,7 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
                cycles = __arch_get_hw_counter(vd->clock_mode);
                ns = vdso_ts->nsec;
                last = vd->cycle_last;
-               if (unlikely((s64)cycles < 0))
+               if (unlikely(cycles == U64_MAX))
                        return -1;
 
                ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
-- 
2.7.0