[PATCH 1/4] x86/fpu: document user_fpu_begin()

Oleg Nesterov
2015-03-11T17:36:19Z

Currently user_fpu_begin() has a single caller and it is not clear that
why do we actually need it, and why we should not worry about preemption
right after preempt_enable().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/include/asm/fpu-internal.h | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 4bec98f..c615ae9 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -464,7 +464,9 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
* Need to be preemption-safe.
*
* NOTE! user_fpu_begin() must be used only immediately before restoring
- * it. This function does not do any save/restore on their own.
+ * it. This function does not do any save/restore on its own. In a lazy
+ * fpu mode this is just optimization to avoid a dna fault, the task can
+ * lose FPU right after preempt_enable().
*/
static inline void user_fpu_begin(void)
{
--
1.5.5.1