[tip:x86/fpu] x86/fpu: Fix fpu__activate_fpstate_read() and update comments

[tip-bot for Ingo Molnar]

Commit-ID:  4618e90965f272fe522f2af2523a60d0d4bc78f3
Gitweb:     http://git.kernel.org/tip/4618e90965f272fe522f2af2523a60d0d4bc78f3
Author:     Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate: Sat, 23 Sep 2017 15:00:10 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 26 Sep 2017 09:41:09 +0200

x86/fpu: Fix fpu__activate_fpstate_read() and update comments

fpu__activate_fpstate_read() can be called for the current task
when coredumping - or for stopped tasks when ptrace-ing.

Implement this properly in the code and update the comments.

This also fixes an incorrect (but harmless) warning introduced by
one of the earlier patches.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Eric Biggers <ebiggers3@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20170923130016.21448-28-mingo@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/fpu/core.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 93103a9..afd3f2a 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -254,18 +254,21 @@ EXPORT_SYMBOL_GPL(fpu__activate_curr);
 /*
  * This function must be called before we read a task's fpstate.
  *
- * If the task has not used the FPU before then initialize its
- * fpstate.
+ * There's two cases where this gets called:
+ *
+ * - for the current task (when coredumping), in which case we have
+ *   to save the latest FPU registers into the fpstate,
+ *
+ * - or it's called for stopped tasks (ptrace), in which case the
+ *   registers were already saved by the context-switch code when
+ *   the task scheduled out - we only have to initialize the registers
+ *   if they've never been initialized.
  *
  * If the task has used the FPU before then save it.
  */
 void fpu__activate_fpstate_read(struct fpu *fpu)
 {
-	/*
-	 * If fpregs are active (in the current CPU), then
-	 * copy them to the fpstate:
-	 */
-	if (fpu->fpstate_active) {
+	if (fpu == &current->thread.fpu) {
 		fpu__save(fpu);
 	} else {
 		if (!fpu->fpstate_active) {