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

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Date:   Mon, 23 Dec 2019 14:31:08 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, arnd@...db.de,
        tglx@...utronix.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: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in
 __c_kernel_time()

READ_ONCE() forces the read of the 64 bit value of
vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec allthough
only the lower part is needed.

This results in a suboptimal code:

00000af4 <__c_kernel_time>:
 af4:	2c 03 00 00 	cmpwi   r3,0
 af8:	81 44 00 20 	lwz     r10,32(r4)
 afc:	81 64 00 24 	lwz     r11,36(r4)
 b00:	41 82 00 08 	beq     b08 <__c_kernel_time+0x14>
 b04:	91 63 00 00 	stw     r11,0(r3)
 b08:	7d 63 5b 78 	mr      r3,r11
 b0c:	4e 80 00 20 	blr

By removing the READ_ONCE(), only the lower part is read from
memory, and the code is cleaner:

00000af4 <__c_kernel_time>:
 af4:	7c 69 1b 79 	mr.     r9,r3
 af8:	80 64 00 24 	lwz     r3,36(r4)
 afc:	4d 82 00 20 	beqlr
 b00:	90 69 00 00 	stw     r3,0(r9)
 b04:	4e 80 00 20 	blr

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 lib/vdso/gettimeofday.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 17b4cff6e5f0..5a17a9d2e6cd 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -144,7 +144,7 @@ __cvdso_gettimeofday(const struct vdso_data *vd, struct __kernel_old_timeval *tv
 static __maybe_unused __kernel_old_time_t
 __cvdso_time(const struct vdso_data *vd, __kernel_old_time_t *time)
 {
-	__kernel_old_time_t t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
+	__kernel_old_time_t t = vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec;
 
 	if (time)
 		*time = t;
-- 
2.13.3