[PATCH 9/9] ftrace: Reverse what the RECURSION flag means in the ftrace_ops

Steven Rostedt
2020-10-29T00:56:16Z

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Now that all callbacks are recursion safe, reverse the meaning of the
RECURSION flag and rename it from RECURSION_SAFE to simply RECURSION.
Now only callbacks that request to have recursion protecting it will
have the added trampoline to do so.

Also remove the outdated comment about "PER_CPU" when determining to
use the ftrace_ops_assist_func.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Documentation/trace/ftrace-uses.rst | 82 +++++++++++++++++++++--------
include/linux/ftrace.h | 12 ++---
kernel/trace/fgraph.c | 3 +-
kernel/trace/ftrace.c | 20 ++++---
kernel/trace/trace_events.c | 1 -
kernel/trace/trace_functions.c | 2 +-
kernel/trace/trace_selftest.c | 7 +--
kernel/trace/trace_stack.c | 1 -
8 files changed, 79 insertions(+), 49 deletions(-)

diff --git a/Documentation/trace/ftrace-uses.rst b/Documentation/trace/ftrace-uses.rst
index 2a05e770618a..cce218d65f40 100644
--- a/Documentation/trace/ftrace-uses.rst
+++ b/Documentation/trace/ftrace-uses.rst
@@ -30,8 +30,8 @@ The ftrace context
This requires extra care to what can be done inside a callback. A callback
can be called outside the protective scope of RCU.

-The ftrace infrastructure has some protections against recursions and RCU
-but one must still be very careful how they use the callbacks.
+There are helper functions to help against recursion, and making sure
+RCU is watching. These are explained below.

The ftrace_ops structure
@@ -108,6 +108,50 @@ The prototype of the callback function is as follows (as of v4.14):
at the start of the function where ftrace was tracing. Otherwise it
either contains garbage, or NULL.

+Protect your callback
+=====================
+
+As functions can be called from anywhere, and it is possible that a function
+called by a callback may also be traced, and call that same callback,
+recursion protection must be used. There are two helper functions that
+can help in this regard. If you start your code with:
+
+ int bit;
+
+ bit = ftrace_test_recursion_trylock();
+ if (bit < 0)
+ return;
+
+and end it with:
+
+ ftrace_test_recursion_unlock(bit);
+
+The code in between will be safe to use, even if it ends up calling a
+function that the callback is tracing. Note, on success,
+ftrace_test_recursion_trylock() will disable preemption, and the
+ftrace_test_recursion_unlock() will enable it again (if it was previously
+enabled).
+
+Alternatively, if the FTRACE_OPS_FL_RECURSION flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for recursion for the callback and no recursion test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+
+If your callback accesses any data or critical section that requires RCU
+protection, it is best to make sure that RCU is "watching", otherwise
+that data or critical section will not be protected as expected. In this
+case add:
+
+ if (!rcu_is_watching())
+ return;
+
+Alternatively, if the FTRACE_OPS_FL_RCU flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for rcu_is_watching for the callback and no other test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+

The ftrace FLAGS
================
@@ -128,26 +172,20 @@ FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
will not fail with this flag set. But the callback must check if
regs is NULL or not to determine if the architecture supports it.

-FTRACE_OPS_FL_RECURSION_SAFE
- By default, a wrapper is added around the callback to
- make sure that recursion of the function does not occur. That is,
- if a function that is called as a result of the callback's execution
- is also traced, ftrace will prevent the callback from being called
- again. But this wrapper adds some overhead, and if the callback is
- safe from recursion, it can set this flag to disable the ftrace
- protection.
-
- Note, if this flag is set, and recursion does occur, it could cause
- the system to crash, and possibly reboot via a triple fault.
-
- It is OK if another callback traces a function that is called by a
- callback that is marked recursion safe. Recursion safe callbacks
- must never trace any function that are called by the callback
- itself or any nested functions that those functions call.
-
- If this flag is set, it is possible that the callback will also
- be called with preemption enabled (when CONFIG_PREEMPTION is set),
- but this is not guaranteed.
+FTRACE_OPS_FL_RECURSION
+ By default, it is expected that the callback can handle recursion.
+ But if the callback is not that worried about overehead, then
+ setting this bit will add the recursion protection around the
+ callback by calling a helper function that will do the recursion
+ protection and only call the callback if it did not recurse.
+
+ Note, if this flag is not set, and recursion does occur, it could
+ cause the system to crash, and possibly reboot via a triple fault.
+
+ Not, if this flag is set, then the callback will always be called
+ with preemption disabled. If it is not set, then it is possible
+ (but not guaranteed) that the callback will be called in
+ preemptable context.

FTRACE_OPS_FL_IPMODIFY
Requires FTRACE_OPS_FL_SAVE_REGS set. If the callback is to "hijack"
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 0e4164a7f56d..806196345c3f 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -98,7 +98,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
/*
* FTRACE_OPS_FL_* bits denote the state of ftrace_ops struct and are
* set in the flags member.
- * CONTROL, SAVE_REGS, SAVE_REGS_IF_SUPPORTED, RECURSION_SAFE, STUB and
+ * CONTROL, SAVE_REGS, SAVE_REGS_IF_SUPPORTED, RECURSION, STUB and
* IPMODIFY are a kind of attribute flags which can be set only before
* registering the ftrace_ops, and can not be modified while registered.
* Changing those attribute flags after registering ftrace_ops will
@@ -121,10 +121,10 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
* passing regs to the handler.
* Note, if this flag is set, the SAVE_REGS flag will automatically
* get set upon registering the ftrace_ops, if the arch supports it.
- * RECURSION_SAFE - The ftrace_ops can set this to tell the ftrace infrastructure
- * that the call back has its own recursion protection. If it does
- * not set this, then the ftrace infrastructure will add recursion
- * protection for the caller.
+ * RECURSION - The ftrace_ops can set this to tell the ftrace infrastructure
+ * that the call back needs recursion protection. If it does
+ * not set this, then the ftrace infrastructure will assume
+ * that the callback can handle recursion on its own.
* STUB - The ftrace_ops is just a place holder.
* INITIALIZED - The ftrace_ops has already been initialized (first use time
* register_ftrace_function() is called, it will initialized the ops)
@@ -156,7 +156,7 @@ enum {
FTRACE_OPS_FL_DYNAMIC = BIT(1),
FTRACE_OPS_FL_SAVE_REGS = BIT(2),
FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED = BIT(3),
- FTRACE_OPS_FL_RECURSION_SAFE = BIT(4),
+ FTRACE_OPS_FL_RECURSION = BIT(4),
FTRACE_OPS_FL_STUB = BIT(5),
FTRACE_OPS_FL_INITIALIZED = BIT(6),
FTRACE_OPS_FL_DELETED = BIT(7),
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 5658f13037b3..73edb9e4f354 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -334,8 +334,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,

static struct ftrace_ops graph_ops = {
.func = ftrace_stub,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE |
- FTRACE_OPS_FL_INITIALIZED |
+ .flags = FTRACE_OPS_FL_INITIALIZED |
FTRACE_OPS_FL_PID |
FTRACE_OPS_FL_STUB,
#ifdef FTRACE_GRAPH_TRAMP_ADDR
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 4833b6a82ce7..2dcae8251104 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -80,7 +80,7 @@ enum {

struct ftrace_ops ftrace_list_end __read_mostly = {
.func = ftrace_stub,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_STUB,
+ .flags = FTRACE_OPS_FL_STUB,
INIT_OPS_HASH(ftrace_list_end)
};

@@ -866,7 +866,7 @@ static void unregister_ftrace_profiler(void)
#else
static struct ftrace_ops ftrace_profile_ops __read_mostly = {
.func = function_profile_call,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_INITIALIZED,
+ .flags = FTRACE_OPS_FL_INITIALIZED,
INIT_OPS_HASH(ftrace_profile_ops)
};

@@ -1040,8 +1040,7 @@ struct ftrace_ops global_ops = {
.local_hash.notrace_hash = EMPTY_HASH,
.local_hash.filter_hash = EMPTY_HASH,
INIT_OPS_HASH(global_ops)
- .flags = FTRACE_OPS_FL_RECURSION_SAFE |
- FTRACE_OPS_FL_INITIALIZED |
+ .flags = FTRACE_OPS_FL_INITIALIZED |
FTRACE_OPS_FL_PID,
};

@@ -2382,7 +2381,7 @@ static void call_direct_funcs(unsigned long ip, unsigned long pip,

struct ftrace_ops direct_ops = {
.func = call_direct_funcs,
- .flags = FTRACE_OPS_FL_IPMODIFY | FTRACE_OPS_FL_RECURSION_SAFE
+ .flags = FTRACE_OPS_FL_IPMODIFY
| FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS
| FTRACE_OPS_FL_PERMANENT,
/*
@@ -6864,8 +6863,7 @@ void ftrace_init_trace_array(struct trace_array *tr)

struct ftrace_ops global_ops = {
.func = ftrace_stub,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE |
- FTRACE_OPS_FL_INITIALIZED |
+ .flags = FTRACE_OPS_FL_INITIALIZED |
FTRACE_OPS_FL_PID,
};

@@ -7025,11 +7023,11 @@ NOKPROBE_SYMBOL(ftrace_ops_assist_func);
ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops)
{
/*
- * If the function does not handle recursion, needs to be RCU safe,
- * or does per cpu logic, then we need to call the assist handler.
+ * If the function does not handle recursion or needs to be RCU safe,
+ * then we need to call the assist handler.
*/
- if (!(ops->flags & FTRACE_OPS_FL_RECURSION_SAFE) ||
- ops->flags & FTRACE_OPS_FL_RCU)
+ if (ops->flags & (FTRACE_OPS_FL_RECURSION |
+ FTRACE_OPS_FL_RCU))
return ftrace_ops_assist_func;

return ops->func;
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index e705f06c68c6..4a9a2a9853bc 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3712,7 +3712,6 @@ function_test_events_call(unsigned long ip, unsigned long parent_ip,
static struct ftrace_ops trace_ops __initdata =
{
.func = function_test_events_call,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static __init void event_trace_self_test_with_function(void)
diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 943756c01190..89c414ce1388 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -48,7 +48,7 @@ int ftrace_allocate_ftrace_ops(struct trace_array *tr)

/* Currently only the non stack version is supported */
ops->func = function_trace_call;
- ops->flags = FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_PID;
+ ops->flags = FTRACE_OPS_FL_PID;

tr->ops = ops;
ops->private = tr;
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index b5e3496cf803..50dd913d23e7 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -150,17 +150,14 @@ static void trace_selftest_test_dyn_func(unsigned long ip,

static struct ftrace_ops test_probe1 = {
.func = trace_selftest_test_probe1_func,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static struct ftrace_ops test_probe2 = {
.func = trace_selftest_test_probe2_func,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static struct ftrace_ops test_probe3 = {
.func = trace_selftest_test_probe3_func,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static void print_counts(void)
@@ -448,11 +445,11 @@ static void trace_selftest_test_recursion_safe_func(unsigned long ip,

static struct ftrace_ops test_rec_probe = {
.func = trace_selftest_test_recursion_func,
+ .flags = FTRACE_OPS_FL_RECURSION,
};

static struct ftrace_ops test_recsafe_probe = {
.func = trace_selftest_test_recursion_safe_func,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static int
@@ -556,7 +553,7 @@ static void trace_selftest_test_regs_func(unsigned long ip,

static struct ftrace_ops test_regs_probe = {
.func = trace_selftest_test_regs_func,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_SAVE_REGS,
+ .flags = FTRACE_OPS_FL_SAVE_REGS,
};

static int
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index c408423e5d65..969db526a563 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -318,7 +318,6 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip,
static struct ftrace_ops trace_ops __read_mostly =
{
.func = stack_trace_call,
- .flags = FTRACE_OPS_FL_RECURSION_SAFE,
};

static ssize_t
--
2.28.0