linux-kernel - Surprising code generated for vdso_read

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Message-ID: <bd4557a7-9715-59aa-5d8e-488c5e516a98@c-s.fr>
Date:   Thu, 9 Jan 2020 17:52:34 +0000
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, arnd@...db.de,
        tglx@...utronix.de, vincenzo.frascino@....com, luto@...nel.org,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     x86@...nel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org
Subject: Surprising code generated for vdso_read_begin()

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 ?

Christophe