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Date:	Fri, 8 Aug 2014 02:19:37 -0700
From:	tip-bot for Masami Hiramatsu <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, ananth@...ibm.com, hpa@...or.com,
	mingo@...nel.org, masami.hiramatsu.pt@...achi.com,
	tglx@...utronix.de, davem@...emloft.net
Subject: [tip:perf/urgent] kprobes:
  Skip kretprobe hit in NMI context to avoid deadlock

Commit-ID:  f96f56780ca584930bb3a2769d73fd9a101bcbbe
Gitweb:     http://git.kernel.org/tip/f96f56780ca584930bb3a2769d73fd9a101bcbbe
Author:     Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
AuthorDate: Mon, 4 Aug 2014 03:10:16 +0000
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 8 Aug 2014 10:38:04 +0200

kprobes: Skip kretprobe hit in NMI context to avoid deadlock

Skip kretprobe hit in NMI context, because if an NMI happens
inside the critical section protected by kretprobe_table.lock
and another(or same) kretprobe hit, pre_kretprobe_handler
tries to lock kretprobe_table.lock again.
Normal interrupts have no problem because they are disabled
with the lock.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Acked-by: Ananth N Mavinakayanahalli <ananth@...ibm.com>
Cc: David S. Miller <davem@...emloft.net>
Link: http://lkml.kernel.org/r/20140804031016.11433.65539.stgit@kbuild-fedora.novalocal
[ Minor edits for clarity. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/kprobes.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 734e9a7..3995f54 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1778,7 +1778,18 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 	unsigned long hash, flags = 0;
 	struct kretprobe_instance *ri;
 
-	/*TODO: consider to only swap the RA after the last pre_handler fired */
+	/*
+	 * To avoid deadlocks, prohibit return probing in NMI contexts,
+	 * just skip the probe and increase the (inexact) 'nmissed'
+	 * statistical counter, so that the user is informed that
+	 * something happened:
+	 */
+	if (unlikely(in_nmi())) {
+		rp->nmissed++;
+		return 0;
+	}
+
+	/* TODO: consider to only swap the RA after the last pre_handler fired */
 	hash = hash_ptr(current, KPROBE_HASH_BITS);
 	raw_spin_lock_irqsave(&rp->lock, flags);
 	if (!hlist_empty(&rp->free_instances)) {
--
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