linux-kernel - [PATCH v2] x86/nmi: remove the irqwork from long duration nmi handler

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Date:   Sat, 11 Jan 2020 20:54:27 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Changbin Du <changbin.du@...il.com>
Subject: [PATCH v2] x86/nmi: remove the irqwork from long duration nmi handler

First, printk is NMI context safe now since the safe printk has been
implemented. The safe printk already has an irqwork to make NMI context
safe.

Second, the NMI irqwork actually does not work if a NMI handler causes
panic by watchdog timeout. This NMI irqwork have no chance to run in such
case, while the safe printk will flush its per-cpu buffer before panic.

While at it, repurpose the IRQ work callback into a function which
concentrates the NMI duration checking.

Signed-off-by: Changbin Du <changbin.du@...il.com>

---
v2: polish commit message. (Borislav Petkov)

---
 arch/x86/include/asm/nmi.h |  1 -
 arch/x86/kernel/nmi.c      | 20 +++++++++-----------
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/nmi.h b/arch/x86/include/asm/nmi.h
index 75ded1d13d98..9d5d949e662e 100644
--- a/arch/x86/include/asm/nmi.h
+++ b/arch/x86/include/asm/nmi.h
@@ -41,7 +41,6 @@ struct nmiaction {
 	struct list_head	list;
 	nmi_handler_t		handler;
 	u64			max_duration;
-	struct irq_work		irq_work;
 	unsigned long		flags;
 	const char		*name;
 };
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index e676a9916c49..aa15d4f2340f 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -104,18 +104,22 @@ static int __init nmi_warning_debugfs(void)
 }
 fs_initcall(nmi_warning_debugfs);
 
-static void nmi_max_handler(struct irq_work *w)
+static void nmi_check_duration(struct nmiaction *action, u64 duration)
 {
-	struct nmiaction *a = container_of(w, struct nmiaction, irq_work);
 	int remainder_ns, decimal_msecs;
-	u64 whole_msecs = READ_ONCE(a->max_duration);
+	u64 whole_msecs = READ_ONCE(action->max_duration);
+
+	if (duration < nmi_longest_ns || duration < action->max_duration)
+		return;
+
+	action->max_duration = duration;
 
 	remainder_ns = do_div(whole_msecs, (1000 * 1000));
 	decimal_msecs = remainder_ns / 1000;
 
 	printk_ratelimited(KERN_INFO
 		"INFO: NMI handler (%ps) took too long to run: %lld.%03d msecs\n",
-		a->handler, whole_msecs, decimal_msecs);
+		action->handler, whole_msecs, decimal_msecs);
 }
 
 static int nmi_handle(unsigned int type, struct pt_regs *regs)
@@ -142,11 +146,7 @@ static int nmi_handle(unsigned int type, struct pt_regs *regs)
 		delta = sched_clock() - delta;
 		trace_nmi_handler(a->handler, (int)delta, thishandled);
 
-		if (delta < nmi_longest_ns || delta < a->max_duration)
-			continue;
-
-		a->max_duration = delta;
-		irq_work_queue(&a->irq_work);
+		nmi_check_duration(a, delta);
 	}
 
 	rcu_read_unlock();
@@ -164,8 +164,6 @@ int __register_nmi_handler(unsigned int type, struct nmiaction *action)
 	if (!action->handler)
 		return -EINVAL;
 
-	init_irq_work(&action->irq_work, nmi_max_handler);
-
 	raw_spin_lock_irqsave(&desc->lock, flags);
 
 	/*
-- 
2.24.0