From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Thomas Gleixner <tglx@linutronix.de>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
x86@kernel.org
Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()
Date: Sat, 11 Jan 2020 08:05:34 +0000 [thread overview]
Message-ID: <a995445f-9b00-ca13-d23a-1aea3b345718@c-s.fr> (raw)
In-Reply-To: <87lfqfrp7d.fsf@nanos.tec.linutronix.de>
On 01/10/2020 09:12 PM, Thomas Gleixner wrote:
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>
>> 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;
>
> Allows the compiler to load twice, i.e. the returned value might be different from the
> stored value. So no.
>
With READ_ONCE() the 64 bits are being read:
00000ac8 <__c_kernel_time>:
ac8: 2c 03 00 00 cmpwi r3,0
acc: 81 44 00 20 lwz r10,32(r4)
ad0: 81 64 00 24 lwz r11,36(r4)
ad4: 41 82 00 08 beq adc <__c_kernel_time+0x14>
ad8: 91 63 00 00 stw r11,0(r3)
adc: 7d 63 5b 78 mr r3,r11
ae0: 4e 80 00 20 blr
Without the READ_ONCE() only 32 bits are read. That's the most optimal.
00000ac8 <__c_kernel_time>:
ac8: 7c 69 1b 79 mr. r9,r3
acc: 80 64 00 24 lwz r3,36(r4)
ad0: 4d 82 00 20 beqlr
ad4: 90 69 00 00 stw r3,0(r9)
ad8: 4e 80 00 20 blr
Without READ_ONCE() but with a barrier() after the read, we should get
the same result but GCC (GCC 8.1) does less good:
00000ac8 <__c_kernel_time>:
ac8: 81 24 00 24 lwz r9,36(r4)
acc: 2f 83 00 00 cmpwi cr7,r3,0
ad0: 41 9e 00 08 beq cr7,ad8 <__c_kernel_time+0x10>
ad4: 91 23 00 00 stw r9,0(r3)
ad8: 7d 23 4b 78 mr r3,r9
adc: 4e 80 00 20 blr
Assuming both part of the 64 bits data will fall into a single
cacheline, the second read is in the noise.
So agreed to drop this change.
Christophe
next prev parent reply other threads:[~2020-01-11 8:06 UTC|newest]
Thread overview: 45+ 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 ` [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() Christophe Leroy
2019-12-24 1:58 ` 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 [this message]
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
2020-01-09 17:52 ` Surprising code generated for vdso_read_begin() Christophe Leroy
2020-01-09 20:07 ` 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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