linux-kernel - [PATCH v4 6/7] arm64: entry: centralize preemption decision

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Message-Id: <20220214165216.2231574-7-mark.rutland@arm.com>
Date:   Mon, 14 Feb 2022 16:52:15 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     ardb@...nel.org, bp@...en8.de, catalin.marinas@....com,
        dave.hansen@...ux.intel.com, frederic@...nel.org,
        james.morse@....com, joey.gouly@....com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        mark.rutland@....com, mingo@...hat.com, peterz@...radead.org,
        tglx@...utronix.de, valentin.schneider@....com, will@...nel.org
Subject: [PATCH v4 6/7] arm64: entry: centralize preemption decision

For historical reasons, the decision of whether or not to preempt is
spread across arm64_preempt_schedule_irq() and __el1_irq(), and it would
be clearer if this were all in one place.

Also, arm64_preempt_schedule_irq() calls lockdep_assert_irqs_disabled(),
but this is redundant, as we have a subsequent identical assertion in
__exit_to_kernel_mode(), and preempt_schedule_irq() will
BUG_ON(!irqs_disabled()) anyway.

This patch removes the redundant assertion and centralizes the
preemption decision making within arm64_preempt_schedule_irq().

Other than the slight change to assertion behaviour, there should be no
functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Acked-by: Ard Biesheuvel <ardb@...nel.org>
Acked-by: Catalin Marinas <catalin.marinas@....com>
Cc: James Morse <james.morse@....com>
Cc: Joey Gouly <joey.gouly@....com>
Cc: Valentin Schneider <valentin.schneider@....com>
Cc: Will Deacon <will@...nel.org>
---
 arch/arm64/kernel/entry-common.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index ef7fcefb96bd..2c639b6b676d 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -222,7 +222,16 @@ static void noinstr arm64_exit_el1_dbg(struct pt_regs *regs)
 
 static void __sched arm64_preempt_schedule_irq(void)
 {
-	lockdep_assert_irqs_disabled();
+	if (!IS_ENABLED(CONFIG_PREEMPTION))
+		return;
+
+	/*
+	 * Note: thread_info::preempt_count includes both thread_info::count
+	 * and thread_info::need_resched, and is not equivalent to
+	 * preempt_count().
+	 */
+	if (READ_ONCE(current_thread_info()->preempt_count) != 0)
+		return;
 
 	/*
 	 * DAIF.DA are cleared at the start of IRQ/FIQ handling, and when GIC
@@ -438,14 +447,7 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
 	do_interrupt_handler(regs, handler);
 	irq_exit_rcu();
 
-	/*
-	 * Note: thread_info::preempt_count includes both thread_info::count
-	 * and thread_info::need_resched, and is not equivalent to
-	 * preempt_count().
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPTION) &&
-	    READ_ONCE(current_thread_info()->preempt_count) == 0)
-		arm64_preempt_schedule_irq();
+	arm64_preempt_schedule_irq();
 
 	exit_to_kernel_mode(regs);
 }
-- 
2.30.2