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SubjectRe: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()
Date
Christophe Leroy <christophe.leroy@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;

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