[PATCH 5/3] x86/fpu: Make sure x86_task_fpu() doesn't get called for PF_KTHREAD tasks during exit

[Ingo Molnar]

fpu__drop() calls x86_task_fpu() unconditionally, while the FPU context
area will not be present if it's the init task, and should not be in
use when it's some other type of kthread.

Return early for PF_KTHREAD tasks. The debug warning in x86_task_fpu()
will catch any kthreads attempting to use the FPU save area.

Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/fpu/core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 3223eb3dd09d..db608afa686f 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -681,7 +681,13 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
  */
 void fpu__drop(struct task_struct *tsk)
 {
-	struct fpu *fpu = x86_task_fpu(tsk);
+	struct fpu *fpu;
+
+	/* PF_KTHREAD tasks do not use the FPU context area: */
+	if (tsk->flags & PF_KTHREAD)
+		return;
+
+	fpu = x86_task_fpu(tsk);
 
 	preempt_disable();