Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

[Thomas Gleixner]

Christophe Leroy <christophe.le...@c-s.fr> writes:
>
> With READ_ONCE() the 64 bits are being read:
>
> Without the READ_ONCE() only 32 bits are read. That's the most optimal.
>
> Without READ_ONCE() but with a barrier() after the read, we should get 
> the same result but GCC (GCC 8.1) does less good:
>
> Assuming both part of the 64 bits data will fall into a single 
> cacheline, the second read is in the noise.

They definitely are in the same cacheline.

> So agreed to drop this change.

We could be smart about this and force the compiler to issue a 32bit
read for 32bit builds. See below. Not sure whether it's worth it, but
OTOH it will take quite a while until the 32bit time interfaces die
completely.

Thanks,

        tglx

8<------------
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -21,6 +21,18 @@
 #define CS_RAW         1
 #define CS_BASES       (CS_RAW + 1)
 
+#ifdef __LITTLE_ENDIAN
+struct sec_hl {
+       u32     sec_l;
+       u32     sec_h;
+};
+#else
+struct sec_hl {
+       u32     sec_h;
+       u32     sec_l;
+};
+#endif
+
 /**
  * struct vdso_timestamp - basetime per clock_id
  * @sec:       seconds
@@ -35,7 +47,10 @@
  * vdso_data.cs[x].shift.
  */
 struct vdso_timestamp {
-       u64     sec;
+       union {
+               u64             sec;
+               struct sec_hl   sec_hl;
+       };
        u64     nsec;
 };
 
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -165,8 +165,13 @@ static __maybe_unused int
 static __maybe_unused __kernel_old_time_t __cvdso_time(__kernel_old_time_t 
*time)
 {
        const struct vdso_data *vd = __arch_get_vdso_data();
-       __kernel_old_time_t t = 
READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
+       __kernel_old_time_t t;
 
+#if BITS_PER_LONG == 32
+       t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec_hl.sec_l);
+#else
+       t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
+#endif
        if (time)
                *time = t;