From: Palmer Dabbelt <palmer@dabbelt.com>
To: linux-riscv@lists.infradead.org
Cc: rostedt@goodmis.org, mingo@redhat.com,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
aou@eecs.berkeley.edu, mhiramat@kernel.org, zong.li@sifive.com,
guoren@linux.alibaba.com, Atish Patra <Atish.Patra@wdc.com>,
linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
kernel-team@android.com,
Palmer Dabbelt <palmerdabbelt@google.com>,
Changbin Du <changbin.du@gmail.com>
Subject: [PATCH] RISC-V: Don't check text_mutex during stop_machine
Date: Thu, 6 May 2021 00:10:41 -0700 [thread overview]
Message-ID: <20210506071041.417854-1-palmer@dabbelt.com> (raw)
From: Palmer Dabbelt <palmerdabbelt@google.com>
We're currently using stop_machine() to update ftrace, which means that
the thread that takes text_mutex during ftrace_prepare() may not be the
same as the thread that eventually patches the code. This isn't
actually a race because the lock is still held (preventing any other
concurrent accesses) and there is only one thread running during
stop_machine(), but it does trigger a lockdep failure.
This patch just elides the lockdep check during stop_machine.
Fixes: c15ac4fd60d5 ("riscv/ftrace: Add dynamic function tracer support")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
---
In theory we should be able to avoid using stop_machine() with some
clever code sequences, but that's too big of a change to be considered a
fix. I also can't find the text I thought was in the ISA manual about
the allowed behaviors for concurrent modification of the instruction
stream, so I might have just mis-remembered that.
---
arch/riscv/include/asm/ftrace.h | 4 ++++
arch/riscv/kernel/ftrace.c | 15 +++++++++++++++
arch/riscv/kernel/patch.c | 10 +++++++++-
3 files changed, 28 insertions(+), 1 deletion(-)
diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
index 04dad3380041..ee8f07b4dbee 100644
--- a/arch/riscv/include/asm/ftrace.h
+++ b/arch/riscv/include/asm/ftrace.h
@@ -85,4 +85,8 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
#endif
+#ifndef __ASSEMBLY__
+extern int riscv_ftrace_in_stop_machine;
+#endif
+
#endif /* _ASM_RISCV_FTRACE_H */
diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 7f1e5203de88..da2405652f1d 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -11,6 +11,8 @@
#include <asm/cacheflush.h>
#include <asm/patch.h>
+int riscv_ftrace_in_stop_machine;
+
#ifdef CONFIG_DYNAMIC_FTRACE
int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
{
@@ -232,3 +234,16 @@ int ftrace_disable_ftrace_graph_caller(void)
}
#endif /* CONFIG_DYNAMIC_FTRACE */
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
+
+void arch_ftrace_update_code(int command)
+{
+ /*
+ * The code sequences we use for ftrace can't be patched while the
+ * kernel is running, so we need to use stop_machine() to modify them
+ * for now. This doesn't play nice with text_mutex, we use this flag
+ * to elide the check.
+ */
+ riscv_ftrace_in_stop_machine = true;
+ ftrace_run_stop_machine(command);
+ riscv_ftrace_in_stop_machine = false;
+}
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 0b552873a577..7983dba477f0 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -11,6 +11,7 @@
#include <asm/kprobes.h>
#include <asm/cacheflush.h>
#include <asm/fixmap.h>
+#include <asm/ftrace.h>
#include <asm/patch.h>
struct patch_insn {
@@ -59,8 +60,15 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
* Before reaching here, it was expected to lock the text_mutex
* already, so we don't need to give another lock here and could
* ensure that it was safe between each cores.
+ *
+ * We're currently using stop_machine() for ftrace, and while that
+ * ensures text_mutex is held before installing the mappings it does
+ * not ensure text_mutex is held by the calling thread. That's safe
+ * but triggers a lockdep failure, so just elide it for that specific
+ * case.
*/
- lockdep_assert_held(&text_mutex);
+ if (!riscv_ftrace_in_stop_machine)
+ lockdep_assert_held(&text_mutex);
if (across_pages)
patch_map(addr + len, FIX_TEXT_POKE1);
--
2.31.1.527.g47e6f16901-goog
next reply other threads:[~2021-05-06 7:11 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-06 7:10 Palmer Dabbelt [this message]
2021-05-06 13:20 ` [PATCH] RISC-V: Don't check text_mutex during stop_machine Steven Rostedt
2021-05-06 13:25 ` Steven Rostedt
2021-05-22 19:32 ` Palmer Dabbelt
2021-05-28 22:21 ` Steven Rostedt
2022-03-22 2:23 Palmer Dabbelt
2022-05-01 14:09 ` Changbin Du
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