From: Oleg Nesterov <oleg@redhat.com>
Date: Sat, 3 May 2025 14:38:56 +0000 (+0200)
Subject: x86/fpu: Check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER in fpu__drop()
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=016a2e6f8ae5ed544ba8fb2b6d78f64ddfd9d01b;p=users%2Fdwmw2%2Flinux.git

x86/fpu: Check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER in fpu__drop()

PF_KTHREAD|PF_USER_WORKER tasks should never clear TIF_NEED_FPU_LOAD,
so the TIF_NEED_FPU_LOAD check should equally filter them out.

And this way an exiting userspace task can avoid the unnecessary "fwait"
if it does context_switch() at least once on its way to exit_thread().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Chang S . Bae <chang.seok.bae@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250503143856.GA9009@redhat.com
---

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 8d674435f1731..fa131299c7da3 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -693,8 +693,7 @@ void fpu__drop(struct task_struct *tsk)
 {
 	struct fpu *fpu;
 
-	/* PF_KTHREAD tasks do not use the FPU context area: */
-	if (tsk->flags & (PF_KTHREAD | PF_USER_WORKER))
+	if (test_tsk_thread_flag(tsk, TIF_NEED_FPU_LOAD))
 		return;
 
 	fpu = x86_task_fpu(tsk);