[tip:x86/fpu] x86/fpu: Don't do __thread_fpu_end() if use_eager_fpu()

From: tip-bot for Oleg Nesterov
Date: Thu Feb 19 2015 - 06:33:05 EST

Commit-ID: 1a2a7f4ec8e3a7ac582dac4d01fcc7e8acd3bb30
Gitweb: http://git.kernel.org/tip/1a2a7f4ec8e3a7ac582dac4d01fcc7e8acd3bb30
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate: Fri, 6 Feb 2015 15:01:59 -0500
Committer: Borislav Petkov <bp@xxxxxxx>
CommitDate: Thu, 19 Feb 2015 11:12:46 +0100

x86/fpu: Don't do __thread_fpu_end() if use_eager_fpu()

unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
Unconditional __thread_fpu_end() is only correct if we know that this
thread can't return to user-mode and use FPU.

Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
and init_fpu(current) can be only called by the coredumping thread via
regset->get(). But it is exported to modules, and imo this should be
fixed anyway.

And if we check use_eager_fpu() we can use __save_fpu() like fpu_copy()
and save_init_fpu() do.

- It seems that even !use_eager_fpu() case doesn't need the unconditional
__thread_fpu_end(), we only need it if __save_init_fpu() returns 0.

- It is still not clear to me if __save_init_fpu() can safely nest with
another save + restore from __kernel_fpu_begin(). If not, we can use
kernel_fpu_disable() to fix the race.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1423252925-14451-3-git-send-email-riel@xxxxxxxxxx
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
arch/x86/kernel/i387.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 4d0db9e..f3ced6f 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -106,8 +106,12 @@ void unlazy_fpu(struct task_struct *tsk)
if (__thread_has_fpu(tsk)) {
- __save_init_fpu(tsk);
- __thread_fpu_end(tsk);
+ if (use_eager_fpu()) {
+ __save_fpu(tsk);
+ } else {
+ __save_init_fpu(tsk);
+ __thread_fpu_end(tsk);
+ }
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