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Subject: [PATCH v2 2/2] ftrace: do CPU checking after preemption disabled
From: =?UTF-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
To: Guo Ren <guoren@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
 Ingo Molnar <mingo@redhat.com>,
 "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
 Helge Deller <deller@gmx.de>, Michael Ellerman <mpe@ellerman.id.au>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>, Paul Walmsley <paul.walmsley@sifive.com>,
 Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>,
 x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
 Josh Poimboeuf <jpoimboe@redhat.com>, Jiri Kosina <jikos@kernel.org>,
 Miroslav Benes <mbenes@suse.cz>, Petr Mladek <pmladek@suse.com>,
 Joe Lawrence <joe.lawrence@redhat.com>,
 Colin Ian King <colin.king@canonical.com>,
 Masami Hiramatsu <mhiramat@kernel.org>,
 "Peter Zijlstra (Intel)" <peterz@infradead.org>,
 Nicholas Piggin <npiggin@gmail.com>, Jisheng Zhang <jszhang@kernel.org>,
 linux-csky@vger.kernel.org, linux-kernel@vger.kernel.org,
 linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
 linux-riscv@lists.infradead.org, live-patching@vger.kernel.org
References: <8c7de46d-9869-aa5e-2bb9-5dbc2eda395e@linux.alibaba.com>
 <1a8e8d73-b508-f90b-0d82-eb2da45a720e@linux.alibaba.com>
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Date: Wed, 13 Oct 2021 11:18:00 +0800
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

With CONFIG_DEBUG_PREEMPT we observed reports like:

  BUG: using smp_processor_id() in preemptible
  caller is perf_ftrace_function_call+0x6f/0x2e0
  CPU: 1 PID: 680 Comm: a.out Not tainted
  Call Trace:
   <TASK>
   dump_stack_lvl+0x8d/0xcf
   check_preemption_disabled+0x104/0x110
   ? optimize_nops.isra.7+0x230/0x230
   ? text_poke_bp_batch+0x9f/0x310
   perf_ftrace_function_call+0x6f/0x2e0
   ...
   __text_poke+0x5/0x620
   text_poke_bp_batch+0x9f/0x310

This telling us the CPU could be changed after task is preempted, and
the checking on CPU before preemption will be invalid.

Since now ftrace_test_recursion_trylock() will help to disable the
preemption, this patch just do the checking after trylock() to address
the issue.

CC: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Abaci <abaci@linux.alibaba.com>
Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
---
 kernel/trace/trace_event_perf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 6aed10e..fba8cb7 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -441,13 +441,13 @@ void perf_trace_buf_update(void *record, u16 type)
 	if (!rcu_is_watching())
 		return;

-	if ((unsigned long)ops->private != smp_processor_id())
-		return;
-
 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
 	if (bit < 0)
 		return;

+	if ((unsigned long)ops->private != smp_processor_id())
+		goto out;
+
 	event = container_of(ops, struct perf_event, ftrace_ops);

 	/*
-- 
1.8.3.1