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Subject[PATCH 4/3] x86/fpu: don't use current->mm to check for a kthread
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 <peterz@infradead.org>
Signed-off-by: Christoph Hellwig <hch@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
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