linux-kernel - [for-next][PATCH 2/8] x86/mm/kmmio: Remove redundant preempt

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Date:   Wed, 14 Dec 2022 09:01:35 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: [for-next][PATCH 2/8] x86/mm/kmmio: Remove redundant preempt_disable()

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

Now that kmmio uses rcu_read_lock_sched_notrace() there's no reason to
call preempt_disable() as the read_lock_sched_notrace() already does that
and is redundant.

This also removes the preempt_enable_no_resched() as the "no_resched()"
portion was bogus as there's no reason to do that.

Link: https://lkml.kernel.org/r/20221212103703.7129cc5d@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "x86@...nel.org" <x86@...nel.org>
Cc: Karol Herbst <karolherbst@...il.com>
Cc: Pekka Paalanen <ppaalanen@...il.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 arch/x86/mm/kmmio.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
index 853c49877c16..9f82019179e1 100644
--- a/arch/x86/mm/kmmio.c
+++ b/arch/x86/mm/kmmio.c
@@ -246,14 +246,13 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 	page_base &= page_level_mask(l);
 
 	/*
-	 * Preemption is now disabled to prevent process switch during
-	 * single stepping. We can only handle one active kmmio trace
+	 * Hold the RCU read lock over single stepping to avoid looking
+	 * up the probe and kmmio_fault_page again. The rcu_read_lock_sched()
+	 * also disables preemption and prevents process switch during
+	 * the single stepping. We can only handle one active kmmio trace
 	 * per cpu, so ensure that we finish it before something else
-	 * gets to run. We also hold the RCU read lock over single
-	 * stepping to avoid looking up the probe and kmmio_fault_page
-	 * again.
+	 * gets to run.
 	 */
-	preempt_disable();
 	rcu_read_lock_sched_notrace();
 
 	faultpage = get_kmmio_fault_page(page_base);
@@ -324,7 +323,6 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 
 no_kmmio:
 	rcu_read_unlock_sched_notrace();
-	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -364,7 +362,6 @@ static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
 	ctx->active--;
 	BUG_ON(ctx->active);
 	rcu_read_unlock_sched_notrace();
-	preempt_enable_no_resched();
 
 	/*
 	 * if somebody else is singlestepping across a probe point, flags
-- 
2.35.1