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Subject: Re: Surprising code generated for vdso_read_begin()
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     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,
        x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mips@vger.kernel.org
References: <cover.1577111363.git.christophe.leroy@c-s.fr>
 <bd4557a7-9715-59aa-5d8e-488c5e516a98@c-s.fr>
 <20200109200733.GS3191@gate.crashing.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr>
Date:   Fri, 10 Jan 2020 07:45:44 +0100
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Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:
>> Wondering why we get something so complicated/redundant for
>> vdso_read_begin() <include/vdso/helpers.h>
>>
>> static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>> {
>> 	u32 seq;
>>
>> 	while ((seq = READ_ONCE(vd->seq)) & 1)
>> 		cpu_relax();
>>
>> 	smp_rmb();
>> 	return seq;
>> }
>>
>>
>>   6e0:   81 05 00 f0     lwz     r8,240(r5)
>>   6e4:   71 09 00 01     andi.   r9,r8,1
>>   6e8:   41 82 00 10     beq     6f8 <__c_kernel_clock_gettime+0x158>
>>   6ec:   81 05 00 f0     lwz     r8,240(r5)
>>   6f0:   71 0a 00 01     andi.   r10,r8,1
>>   6f4:   40 82 ff f8     bne     6ec <__c_kernel_clock_gettime+0x14c>
>>   6f8:
>>
>> r5 being vd pointer
>>
>> Why the first triplet, not only the second triplet ? Something wrong
>> with using READ_ONCE() for that ?
> 
> It looks like the compiler did loop peeling.  What GCC version is this?
> Please try current trunk (to become GCC 10), or at least GCC 9?
> 

It is with GCC 5.5

https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more 
recent than 8.1

With 8.1, the problem doesn't show up.

Thanks
Christophe