i387: fix up some fpu_counter confusion

This makes sure we clear the FPU usage counter for newly created tasks,
just so that we start off in a known state (for example, don't try to
preload the FPU state on the first task switch etc).

It also fixes a thinko in when we increment the fpu_counter at task
switch time, introduced by commit 34ddc81a230b ("i387: re-introduce FPU
state preloading at context switch time").  We should increment the
*new* task fpu_counter, not the old task, and only if we decide to use
that state (whether lazily or preloaded).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 4 insertions, 1 deletions

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index a850b4d8d14d..8df95849721d 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -348,10 +348,10 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
                 if (__save_init_fpu(old))
                         fpu_lazy_state_intact(old);
                 __thread_clear_has_fpu(old);
-                old->fpu_counter++;
 
                 /* Don't change CR0.TS if we just switch! */
                 if (fpu.preload) {
+                        new->fpu_counter++;
                         __thread_set_has_fpu(new);
                         prefetch(new->thread.fpu.state);
                 } else
@@ -359,6 +359,7 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
         } else {
                 old->fpu_counter = 0;
                 if (fpu.preload) {
+                        new->fpu_counter++;
                         if (fpu_lazy_restore(new))
                                 fpu.preload = 0;
                         else
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 80bfe1ab0031..bc32761bc27a 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -214,6 +214,7 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
         task_user_gs(p) = get_user_gs(regs);
 
+        p->fpu_counter = 0;
         p->thread.io_bitmap_ptr = NULL;
         tsk = current;
         err = -ENOMEM;
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 1fd94bc4279d..8ad880b3bc1c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -286,6 +286,7 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
         set_tsk_thread_flag(p, TIF_FORK);
 
+        p->fpu_counter = 0;
         p->thread.io_bitmap_ptr = NULL;
 
         savesegment(gs, p->thread.gsindex);