Subject: Re: Surprising code generated for vdso_read_begin()

From: Segher Boessenkool <[email-masked]>
Date: Thu, 9 Jan 2020 14:07:33 -0600

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?

Segher