[tip:x86/fpu] x86/fpu: Document user_fpu_begin()

[tip-bot for Oleg Nesterov <tipbot@zytor.com>]

Commit-ID:  fb14b4eadf73500d3b2104f031472a268562c047
Gitweb:     http://git.kernel.org/tip/fb14b4eadf73500d3b2104f031472a268562c047
Author:     Oleg Nesterov <oleg@redhat.com>
AuthorDate: Wed, 11 Mar 2015 18:34:09 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 23 Mar 2015 10:13:58 +0100

x86/fpu: Document user_fpu_begin()

Currently, user_fpu_begin() has a single caller and it is not clear 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>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pekka Riikonen <priikone@iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150311173409.GC5032@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/fpu-internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 810f20f..c58c930 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -508,10 +508,12 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
 }
 
 /*
- * Need to be preemption-safe.
+ * Needs 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.
+ * the save state. It does not do any saving/restoring on its own. In
+ * lazy FPU mode, it is just an optimization to avoid a #NM exception,
+ * the task can lose the FPU right after preempt_enable().
  */
 static inline void user_fpu_begin(void)
 {