Re: mips build failures due to commit 8dd928915a73 (mips: fix up obsolete cpu function usage)

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Apr 21, 2015 at 01:45:35PM +0930, Rusty Russell wrote:
> Aaro Koskinen <aaro.koskinen@iki.fi> writes:
> > Hi,
> >
> > On Mon, Apr 20, 2015 at 12:40:28PM -0700, Guenter Roeck wrote:
> >> the upstream kernel fails to build mips:nlm_xlp_defconfig,
> >> mips:nlm_xlp_defconfig, mips:cavium_octeon_defconfig, and possibly
> >> other targets, with errors such as
> >> 
> >> arch/mips/kernel/smp.c:211:2: error:
> >> 	passing argument 2 of 'cpumask_set_cpu' discards 'volatile' qualifier
> >> 	from pointer target type
> >> arch/mips/kernel/process.c:52:2: error:
> >> 	passing argument 2 of 'cpumask_test_cpu' discards 'volatile' qualifier
> >> 	from pointer target type
> >> arch/mips/cavium-octeon/smp.c:242:2: error:
> >> 	passing argument 2 of 'cpumask_clear_cpu' discards 'volatile' qualifier
> >> 	from pointer target type
> >> 
> >> The problem was introduced with commit 8dd928915a73 (" mips: fix up
> >> obsolete cpu function usage"). I would send a patch to fix it, but I
> >> am not sure if removing 'volatile' from the variable declaration(s)
> >> would be a good idea.
> >
> > I think removing volatile from cpu_callin_map declaration should be OK,
> > since test_cpu (only reader) uses test_bit which takes care of it:
> >
> > 	static inline int test_bit(int nr, const volatile unsigned long *addr)
> 
> No, that got replaced too, with cpumask_test_cpu AFAICT.
> 
> You can open-code it, like so:
> 
>         test_bit(0, cpumask_bits(cpu_callin_map));
> 
> But you probably want to put a barrier in that loop instead of relying
> on volatile.
> 
The following might do it. Note that I can not really test it since I don't have
a real mips system, and qemu gets rcu hangs if I enable more than one CPU (I see
that with older kernels as well, so it is not a new problem). Someone will have
to test the patch on a real multi-core system.

Guenter

---
>From 94026cc98a6b7b3567780a5443674c71202e2497 Mon Sep 17 00:00:00 2001
From: Guenter Roeck <linux@roeck-us.net>
Date: Tue, 21 Apr 2015 08:31:01 -0700
Subject: [PATCH] mips: Fix SMP builds

Mips SMP builds fail with error messages similar to the following.

arch/mips/kernel/smp.c:211:2: error:
	passing argument 2 of 'cpumask_set_cpu' discards 'volatile'
	qualifier from pointer target type
arch/mips/kernel/process.c:52:2: error:
	passing argument 2 of 'cpumask_test_cpu' discards 'volatile'
	qualifier from pointer target type
arch/mips/cavium-octeon/smp.c:242:2: error:
	passing argument 2 of 'cpumask_clear_cpu' discards 'volatile'
	qualifier from pointer target type

cpu_callin_map is declared as volatile variable, but passed to various
functions with non-volatile arguments. Make it non-volatile and add a
memory barrier at the one location where volatile might be needed.

Fixes: 8dd928915a73 ("mips: fix up obsolete cpu function usage")
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 arch/mips/include/asm/smp.h | 2 +-
 arch/mips/kernel/smp.c      | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/mips/include/asm/smp.h b/arch/mips/include/asm/smp.h
index bb02fac9b4fa..2b25d1ba1ea0 100644
--- a/arch/mips/include/asm/smp.h
+++ b/arch/mips/include/asm/smp.h
@@ -45,7 +45,7 @@ extern int __cpu_logical_map[NR_CPUS];
 #define SMP_DUMP		0x8
 #define SMP_ASK_C0COUNT		0x10
 
-extern volatile cpumask_t cpu_callin_map;
+extern cpumask_t cpu_callin_map;
 
 /* Mask of CPUs which are currently definitely operating coherently */
 extern cpumask_t cpu_coherent_mask;
diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
index 193ace7955fb..158191394770 100644
--- a/arch/mips/kernel/smp.c
+++ b/arch/mips/kernel/smp.c
@@ -43,7 +43,7 @@
 #include <asm/time.h>
 #include <asm/setup.h>
 
-volatile cpumask_t cpu_callin_map;	/* Bitmask of started secondaries */
+cpumask_t cpu_callin_map;		/* Bitmask of started secondaries */
 
 int __cpu_number_map[NR_CPUS];		/* Map physical to logical */
 EXPORT_SYMBOL(__cpu_number_map);
@@ -218,8 +218,10 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 	/*
 	 * Trust is futile.  We should really have timeouts ...
 	 */
-	while (!cpumask_test_cpu(cpu, &cpu_callin_map))
+	while (!cpumask_test_cpu(cpu, &cpu_callin_map)) {
 		udelay(100);
+		mb();
+	}
 
 	synchronise_count_master(cpu);
 	return 0;
-- 
2.1.0