[tip:sched/core] ia64/sn/hwperf: Replace racy task affinity logic

[tip-bot for Thomas Gleixner]

Commit-ID:  9feb42ac88b516e378b9782e82b651ca5bed95c4
Gitweb:     http://git.kernel.org/tip/9feb42ac88b516e378b9782e82b651ca5bed95c4
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Thu, 6 Apr 2017 14:56:18 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Sat, 15 Apr 2017 12:20:53 +0200

ia64/sn/hwperf: Replace racy task affinity logic

sn_hwperf_op_cpu() which is invoked from an ioctl requires to run code on
the requested cpu. This is achieved by temporarily setting the affinity of
the calling user space thread to the requested CPU and reset it to the
original affinity afterwards.

That's racy vs. CPU hotplug and concurrent affinity settings for that
thread resulting in code executing on the wrong CPU and overwriting the
new affinity setting.

Replace it by using work_on_cpu_safe() which guarantees to run the code on
the requested CPU or to fail in case the CPU is offline.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: linux-ia64@xxxxxxxxxxxxxxx
Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Link: alpine.DEB.2.20.1704122251450.2548@nanos">http://lkml.kernel.org/r/alpine.DEB.2.20.1704122251450.2548@nanos
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/ia64/sn/kernel/sn2/sn_hwperf.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/arch/ia64/sn/kernel/sn2/sn_hwperf.c b/arch/ia64/sn/kernel/sn2/sn_hwperf.c
index 52704f1..55febd6 100644
--- a/arch/ia64/sn/kernel/sn2/sn_hwperf.c
+++ b/arch/ia64/sn/kernel/sn2/sn_hwperf.c
@@ -598,12 +598,17 @@ static void sn_hwperf_call_sal(void *info)
 	op_info->ret = r;
 }
 
+static long sn_hwperf_call_sal_work(void *info)
+{
+	sn_hwperf_call_sal(info);
+	return 0;
+}
+
 static int sn_hwperf_op_cpu(struct sn_hwperf_op_info *op_info)
 {
 	u32 cpu;
 	u32 use_ipi;
 	int r = 0;
-	cpumask_t save_allowed;
 	
 	cpu = (op_info->a->arg & SN_HWPERF_ARG_CPU_MASK) >> 32;
 	use_ipi = op_info->a->arg & SN_HWPERF_ARG_USE_IPI_MASK;
@@ -629,13 +634,9 @@ static int sn_hwperf_op_cpu(struct sn_hwperf_op_info *op_info)
 			/* use an interprocessor interrupt to call SAL */
 			smp_call_function_single(cpu, sn_hwperf_call_sal,
 				op_info, 1);
-		}
-		else {
-			/* migrate the task before calling SAL */ 
-			save_allowed = current->cpus_allowed;
-			set_cpus_allowed_ptr(current, cpumask_of(cpu));
-			sn_hwperf_call_sal(op_info);
-			set_cpus_allowed_ptr(current, &save_allowed);
+		} else {
+			/* Call on the target CPU */
+			work_on_cpu_safe(cpu, sn_hwperf_call_sal_work, op_info);
 		}
 	}
 	r = op_info->ret;