Messages in this thread | | | Subject | Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() | From | Christophe Leroy <> | Date | Sat, 11 Jan 2020 08:05:34 +0000 |
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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
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