Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :

On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:

quoted

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel