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ftrace: Reverse what the RECURSION flag means in the ftrace_ops 

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Also remove the outdated comment about "PER_CPU" when determining to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

Link: https://lkml.kernel.org/r/20201028115613.742454631@goodmis.org
Link: https://lkml.kernel.org/r/20201106023547.904270143@goodmis.org

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh  Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
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>
This commit is contained in:
Steven Rostedt (VMware) 2020-11-05 21:32:45 -05:00
parent 5d029b035b
commit a25d036d93
8 changed files with 77 additions and 47 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

78
Documentation/trace/ftrace-uses.rst
View File

@ -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.
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 set, and recursion does occur, it could cause
the system to crash, and possibly reboot via a triple fault.
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.
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.
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"

12
include/linux/ftrace.h
View File

@ -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),

3
kernel/trace/fgraph.c
View File

@ -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

20
kernel/trace/ftrace.c
View File

@ -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,
};
@ -7023,11 +7021,11 @@ 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;

1
kernel/trace/trace_events.c
View File

@ -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)

2
kernel/trace/trace_functions.c
View File

@ -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;

7
kernel/trace/trace_selftest.c
View File

@ -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
@ -561,7 +558,7 @@ 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

1
kernel/trace/trace_stack.c
View File

@ -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