1 files changed, 42 insertions, 38 deletions

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index 8d85046adcc8..fe6001d748cf 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -29,6 +29,7 @@
 #include <linux/kallsyms.h>
 #include <linux/random.h>
 #include <linux/prctl.h>
+#include <linux/cpu.h>
 
 #include <asm/asm.h>
 #include <asm/bootinfo.h>
@@ -691,19 +692,25 @@ int mips_get_process_fp_mode(struct task_struct *task)
 	return value;
 }
 
-static void prepare_for_fp_mode_switch(void *info)
+static long prepare_for_fp_mode_switch(void *unused)
 {
-	struct mm_struct *mm = info;
-
-	if (current->mm == mm)
-		lose_fpu(1);
+	/*
+	 * This is icky, but we use this to simply ensure that all CPUs have
+	 * context switched, regardless of whether they were previously running
+	 * kernel or user code. This ensures that no CPU currently has its FPU
+	 * enabled, or is about to attempt to enable it through any path other
+	 * than enable_restore_fp_context() which will wait appropriately for
+	 * fp_mode_switching to be zero.
+	 */
+	return 0;
 }
 
 int mips_set_process_fp_mode(struct task_struct *task, unsigned int value)
 {
 	const unsigned int known_bits = PR_FP_MODE_FR | PR_FP_MODE_FRE;
 	struct task_struct *t;
-	int max_users;
+	struct cpumask process_cpus;
+	int cpu;
 
 	/* If nothing to change, return right away, successfully.  */
 	if (value == mips_get_process_fp_mode(task))
@@ -736,35 +743,7 @@ int mips_set_process_fp_mode(struct task_struct *task, unsigned int value)
 	if (!(value & PR_FP_MODE_FR) && raw_cpu_has_fpu && cpu_has_mips_r6)
 		return -EOPNOTSUPP;
 
-	/* Proceed with the mode switch */
-	preempt_disable();
-
-	/* Save FP & vector context, then disable FPU & MSA */
-	if (task->signal == current->signal)
-		lose_fpu(1);
-
-	/* Prevent any threads from obtaining live FP context */
-	atomic_set(&task->mm->context.fp_mode_switching, 1);
-	smp_mb__after_atomic();
-
-	/*
-	 * If there are multiple online CPUs then force any which are running
-	 * threads in this process to lose their FPU context, which they can't
-	 * regain until fp_mode_switching is cleared later.
-	 */
-	if (num_online_cpus() > 1) {
-		/* No need to send an IPI for the local CPU */
-		max_users = (task->mm == current->mm) ? 1 : 0;
-
-		if (atomic_read(&current->mm->mm_users) > max_users)
-			smp_call_function(prepare_for_fp_mode_switch,
-					  (void *)current->mm, 1);
-	}
-
-	/*
-	 * There are now no threads of the process with live FP context, so it
-	 * is safe to proceed with the FP mode switch.
-	 */
+	/* Indicate the new FP mode in each thread */
 	for_each_thread(task, t) {
 		/* Update desired FP register width */
 		if (value & PR_FP_MODE_FR) {
@@ -781,9 +760,34 @@ int mips_set_process_fp_mode(struct task_struct *task, unsigned int value)
 			clear_tsk_thread_flag(t, TIF_HYBRID_FPREGS);
 	}
 
-	/* Allow threads to use FP again */
-	atomic_set(&task->mm->context.fp_mode_switching, 0);
-	preempt_enable();
+	/*
+	 * We need to ensure that all threads in the process have switched mode
+	 * before returning, in order to allow userland to not worry about
+	 * races. We can do this by forcing all CPUs that any thread in the
+	 * process may be running on to schedule something else - in this case
+	 * prepare_for_fp_mode_switch().
+	 *
+	 * We begin by generating a mask of all CPUs that any thread in the
+	 * process may be running on.
+	 */
+	cpumask_clear(&process_cpus);
+	for_each_thread(task, t)
+		cpumask_set_cpu(task_cpu(t), &process_cpus);
+
+	/*
+	 * Now we schedule prepare_for_fp_mode_switch() on each of those CPUs.
+	 *
+	 * The CPUs may have rescheduled already since we switched mode or
+	 * generated the cpumask, but that doesn't matter. If the task in this
+	 * process is scheduled out then our scheduling
+	 * prepare_for_fp_mode_switch() will simply be redundant. If it's
+	 * scheduled in then it will already have picked up the new FP mode
+	 * whilst doing so.
+	 */
+	get_online_cpus();
+	for_each_cpu_and(cpu, &process_cpus, cpu_online_mask)
+		work_on_cpu(cpu, prepare_for_fp_mode_switch, NULL);
+	put_online_cpus();
 
 	wake_up_var(&task->mm->context.fp_mode_switching);