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

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Date:   Sat, 11 Jan 2020 08:05:34 +0000
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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()



On 01/10/2020 09:12 PM, Thomas Gleixner wrote:
> Christophe Leroy <christophe.leroy@....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