From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
arnd@arndb.de, tglx@linutronix.de, vincenzo.frascino@arm.com,
luto@kernel.org
Cc: x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org
Subject: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()
Date: Mon, 23 Dec 2019 14:31:08 +0000 (UTC) [thread overview]
Message-ID: <fc1ff722c7cbe63a63ae02ade3a714d2049d54a5.1577111367.git.christophe.leroy@c-s.fr> (raw)
In-Reply-To: <cover.1577111363.git.christophe.leroy@c-s.fr>
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@c-s.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
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next prev parent reply other threads:[~2019-12-23 14:33 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-23 14:31 [RFC PATCH v2 00/10] powerpc/32: switch VDSO to C implementation Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 01/10] lib: vdso: ensure all arches have 32bit fallback Christophe Leroy
2019-12-24 2:07 ` Andy Lutomirski
2020-01-10 20:56 ` Thomas Gleixner
2020-01-10 21:02 ` Andy Lutomirski
2019-12-30 12:27 ` Arnd Bergmann
2020-01-02 11:29 ` Arnd Bergmann
2020-01-09 15:43 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 02/10] lib: vdso: move call to fallback out of common code Christophe Leroy
2019-12-24 2:24 ` Andy Lutomirski
2019-12-24 11:41 ` christophe leroy
2019-12-24 12:09 ` Andy Lutomirski
2019-12-23 14:31 ` [RFC PATCH v2 03/10] lib: vdso: Change __cvdso_clock_gettime/getres_common() to __cvdso_clock_gettime/getres() Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 04/10] lib: vdso: get pointer to vdso data from the arch Christophe Leroy
2019-12-24 2:27 ` Andy Lutomirski
2019-12-24 11:53 ` christophe leroy
2019-12-24 12:15 ` Andy Lutomirski
2019-12-24 12:41 ` Andy Lutomirski
2019-12-24 14:46 ` Segher Boessenkool
2019-12-23 14:31 ` [RFC PATCH v2 05/10] lib: vdso: inline do_hres() Christophe Leroy
2019-12-24 2:29 ` Andy Lutomirski
2019-12-30 12:07 ` Arnd Bergmann
2020-01-10 21:07 ` Thomas Gleixner
2020-01-11 9:06 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 06/10] lib: vdso: make do_coarse() return 0 Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy [this message]
2019-12-24 1:58 ` [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() Andy Lutomirski
2019-12-24 11:12 ` christophe leroy
2019-12-24 12:04 ` Andy Lutomirski
2020-01-10 21:12 ` Thomas Gleixner
2020-01-11 8:05 ` Christophe Leroy
2020-01-11 11:07 ` Thomas Gleixner
2020-01-13 6:52 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 08/10] lib: vdso: Avoid duplication in __cvdso_clock_getres() Christophe Leroy
2019-12-24 1:59 ` Andy Lutomirski
2019-12-23 14:31 ` [RFC PATCH v2 09/10] powerpc/vdso32: inline __get_datapage() Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 10/10] powerpc/32: Switch VDSO to C implementation Christophe Leroy
[not found] ` <bd4557a7-9715-59aa-5d8e-488c5e516a98@c-s.fr>
2020-01-09 20:07 ` Surprising code generated for vdso_read_begin() Segher Boessenkool
2020-01-10 6:45 ` Christophe Leroy
2020-01-11 11:33 ` Segher Boessenkool
2020-02-16 18:10 ` Arnd Bergmann
2020-02-19 8:45 ` Christophe Leroy
2020-02-19 9:52 ` Arnd Bergmann
2020-02-19 13:08 ` Segher Boessenkool
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