[PATCH 4/3] x86/fpu: don't use current->mm to check for a kthread

[Christoph Hellwig]

current->mm can be non-NULL if a kthread calls use_mm.  Check for
PF_KTHREAD instead to decide when to store user mode FP state.

Fixes: 2722146eb784 ("x86/fpu: Remove fpu->initialized")
Reported-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Christoph Hellwig <h...@lst.de>
---
 arch/x86/include/asm/fpu/internal.h | 6 +++---
 arch/x86/kernel/fpu/core.c          | 3 ++-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h 
b/arch/x86/include/asm/fpu/internal.h
index 9e27fa05a7ae..4c95c365058a 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -536,7 +536,7 @@ static inline void __fpregs_load_activate(void)
        struct fpu *fpu = &current->thread.fpu;
        int cpu = smp_processor_id();
 
-       if (WARN_ON_ONCE(current->mm == NULL))
+       if (WARN_ON_ONCE(current->flags & PF_KTHREAD))
                return;
 
        if (!fpregs_state_valid(fpu, cpu)) {
@@ -567,11 +567,11 @@ static inline void __fpregs_load_activate(void)
  * otherwise.
  *
  * The FPU context is only stored/restored for a user task and
- * ->mm is used to distinguish between kernel and user threads.
+ * PF_KTHREAD is used to distinguish between kernel and user threads.
  */
 static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
-       if (static_cpu_has(X86_FEATURE_FPU) && current->mm) {
+       if (static_cpu_has(X86_FEATURE_FPU) && !(current->flags & PF_KTHREAD)) {
                if (!copy_fpregs_to_fpstate(old_fpu))
                        old_fpu->last_cpu = -1;
                else
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 5e0240d029fd..12c70840980e 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -91,7 +91,8 @@ void kernel_fpu_begin(void)
 
        this_cpu_write(in_kernel_fpu, true);
 
-       if (current->mm && !test_thread_flag(TIF_NEED_FPU_LOAD)) {
+       if (!(current->flags & PF_KTHREAD) &&
+           !test_thread_flag(TIF_NEED_FPU_LOAD)) {
                set_thread_flag(TIF_NEED_FPU_LOAD);
                /*
                 * Ignore return value -- we don't care if reg state
-- 
2.20.1