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Message-ID: <20240610124406.359476013@linutronix.de>
From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
 Sam Sun <samsun1006219@gmail.com>,
 x86@kernel.org,
 syzkaller-bugs@googlegroups.com,
 xrivendell7@gmail.com
Subject: [patch 1/4] perf/x86: Serialize set_attr_rdpmc()
References: <87bk49xf15.ffs@tglx>
 <20240610124258.109097511@linutronix.de>
Precedence: bulk
MIME-Version: 1.0
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Date: Mon, 10 Jun 2024 14:46:35 +0200 (CEST)

Yue and Xingwei reported a jump label failure. It's caused by the lack of
serialization in set_attr_rdpmc():

CPU0                           CPU1

Assume: x86_pmu.attr_rdpmc == 0

if (val != x86_pmu.attr_rdpmc) {
  if (val == 0)
    ...
  else if (x86_pmu.attr_rdpmc == 0)
    static_branch_dec(&rdpmc_never_available_key);

				if (val != x86_pmu.attr_rdpmc) {
				   if (val == 0)
				      ...
				   else if (x86_pmu.attr_rdpmc == 0)
     FAIL, due to imbalance --->      static_branch_dec(&rdpmc_never_available_key);

The reported BUG() is a consequence of the above and of another bug in the
jump label core code. The core code needs a separate fix, but that cannot
prevent the imbalance problem caused by set_attr_rdpmc().

Prevent this by serializing set_attr_rdpmc() locally.

Fixes: a66734297f78 ("perf/x86: Add /sys/devices/cpu/rdpmc=2 to allow rdpmc for all tasks")
Reported-by: Yue Sun <samsun1006219@gmail.com>
Reported-by: Xingwei Lee <xrivendell7@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Closes: https://lore.kernel.org/r/CAEkJfYNzfW1vG=ZTMdz_Weoo=RXY1NDunbxnDaLyj8R4kEoE_w@mail.gmail.com
---
 arch/x86/events/core.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2547,6 +2547,7 @@ static ssize_t set_attr_rdpmc(struct dev
 			      struct device_attribute *attr,
 			      const char *buf, size_t count)
 {
+	static DEFINE_MUTEX(rdpmc_mutex);
 	unsigned long val;
 	ssize_t ret;
 
@@ -2560,6 +2561,8 @@ static ssize_t set_attr_rdpmc(struct dev
 	if (x86_pmu.attr_rdpmc_broken)
 		return -ENOTSUPP;
 
+	guard(mutex)(&rdpmc_mutex);
+
 	if (val != x86_pmu.attr_rdpmc) {
 		/*
 		 * Changing into or out of never available or always available,