[PATCH] ARM: pcpu: ensure __my_cpu_offset cannot be re-ordered across barrier()

[Will Deacon]

On Tue, Jun 04, 2013 at 11:39:53AM +0100, Will Deacon wrote:
> However, if you instead add a dependency on something that already lives in a
> register, like __builtin_return_address, you get:
> 
> c001cd44:       e92d4010        push    {r4, lr}
> c001cd48:       ee1dcf90        mrc     15, 0, ip, cr13, cr0, {4}
> c001cd4c:       e790410c        ldr     r4, [r0, ip, lsl #2]
> c001cd50:       ee1dcf90        mrc     15, 0, ip, cr13, cr0, {4}
> c001cd54:       e791110c        ldr     r1, [r1, ip, lsl #2]
> c001cd58:       e792210c        ldr     r2, [r2, ip, lsl #2]
> c001cd5c:       ee1d3f90        mrc     15, 0, r3, cr13, cr0, {4}
> c001cd60:       e7903103        ldr     r3, [r0, r3, lsl #2]
> c001cd64:       e0840001        add     r0, r4, r1
> c001cd68:       e0800002        add     r0, r0, r2
> c001cd6c:       e0800003        add     r0, r0, r3
> c001cd70:       e8bd8010        pop     {r4, pc}

Actually, I've managed to improve this further using sp instead (see diff
below), which also prevents GCC from moving the lr into other registers for
no reason. The code generated for your test is:

c001cda4:       ee1d3f90        mrc     15, 0, r3, cr13, cr0, {4}
c001cda8:       e790c103        ldr     ip, [r0, r3, lsl #2]
c001cdac:       ee1d3f90        mrc     15, 0, r3, cr13, cr0, {4}
c001cdb0:       e7911103        ldr     r1, [r1, r3, lsl #2]
c001cdb4:       e7922103        ldr     r2, [r2, r3, lsl #2]
c001cdb8:       ee1d3f90        mrc     15, 0, r3, cr13, cr0, {4}
c001cdbc:       e7903103        ldr     r3, [r0, r3, lsl #2]
c001cdc0:       e08c0001        add     r0, ip, r1
c001cdc4:       e0800002        add     r0, r0, r2
c001cdc8:       e0800003        add     r0, r0, r3
c001cdcc:       e12fff1e        bx      lr

I spoke to the tools guys and they'll look into why the original patch
didn't work, since that could have wider implications on our use of
barrier().

Will

--->8

diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h
index 968c0a1..209e650 100644
--- a/arch/arm/include/asm/percpu.h
+++ b/arch/arm/include/asm/percpu.h
@@ -30,8 +30,15 @@ static inline void set_my_cpu_offset(unsigned long off)
 static inline unsigned long __my_cpu_offset(void)
 {
        unsigned long off;
-       /* Read TPIDRPRW */
-       asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : : "memory");
+       register unsigned long *sp asm ("sp");
+
+       /*
+        * Read TPIDRPRW.
+        * We want to allow caching the value, so avoid using volatile and
+        * instead use a fake stack read to hazard against barrier().
+        */
+       asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : "Q" (*sp));
+
        return off;
 }
 #define __my_cpu_offset __my_cpu_offset()