[PATCH 1/4] x86, fpu: introduce per-cpu "bool in_kernel_fpu"

Oleg Nesterov
2014-08-29T18:18:40Z

interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
is safe or not. In particulat it should obviously deny the nested
kernel_fpu_begin() and this logic doesn't look clean.

If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
__thread_clear_has_fpu().

Otherwise we demand that the interrupted task has no FPU if it is in
kernel mode, this works becase __kernel_fpu_begin() does clts().

Add the per-cpu "bool in_kernel_fpu" variable, and change this code
to check/set/clear it. This allows to do some cleanups (see the next
changes) and fixes.

Note that the current code looks racy. Say, kernel_fpu_begin() right
after math_state_restore()->__thread_fpu_begin() will overwrite the
regs we are going to restore. This patch doesn't even try to fix this,
it just adds the comment, but "in_kernel_fpu" can also be used to
implement kernel_fpu_disable() / kernel_fpu_enable().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/include/asm/i387.h | 2 +-
arch/x86/kernel/i387.c | 10 ++++++++++
2 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index ed8089d..5e275d3 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);

static inline void kernel_fpu_begin(void)
{
- WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
+ WARN_ON_ONCE(!irq_fpu_usable());
__kernel_fpu_begin();
}

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index d5dd808..8fb8868 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -19,6 +19,8 @@
#include <asm/fpu-internal.h>
#include <asm/user.h>

+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
/*
* Were we in an interrupt that interrupted kernel mode?
*
@@ -33,6 +35,9 @@
*/
static inline bool interrupted_kernel_fpu_idle(void)
{
+ if (this_cpu_read(in_kernel_fpu))
+ return false;
+
if (use_eager_fpu())
return __thread_has_fpu(current);

@@ -73,6 +78,9 @@ void __kernel_fpu_begin(void)
{
struct task_struct *me = current;

+ this_cpu_write(in_kernel_fpu, true);
+
+ /* FIXME: race with math_state_restore()-like code */
if (__thread_has_fpu(me)) {
__thread_clear_has_fpu(me);
__save_init_fpu(me);
@@ -99,6 +107,8 @@ void __kernel_fpu_end(void)
} else {
stts();
}
+
+ this_cpu_write(in_kernel_fpu, false);
}
EXPORT_SYMBOL(__kernel_fpu_end);

--
1.5.5.1