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

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Message-ID: <878smes13d.fsf@nanos.tec.linutronix.de>
Date:   Sat, 11 Jan 2020 12:07:34 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, arnd@...db.de,
        vincenzo.frascino@....com, luto@...nel.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Christophe Leroy <christophe.leroy@....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;