Messages in this thread | | | Date | Tue, 3 Sep 2013 18:24:04 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC][PATCH 01/18 v2] ftrace: Add hash list to save RCU unsafe functions |
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On Tue, Sep 03, 2013 at 07:57:05PM -0400, Steven Rostedt wrote: > On Tue, 3 Sep 2013 15:18:08 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > > > Just found this bug. Strange that gcc never gave me a warning :-/ > > > > I can't give gcc too much trouble, as I also didn't give you an > > uninitialized-variable warning. > > I was also chasing down a nasty bug that looked to be a pointer > corruption somewhere. Still never found exactly where it happened, but > it always happened with the following conditions: > > synchronize_sched() was in progress > > The ftrace_unsafe_callback() was preempted by an interrupt > > lockdep was processing a lock > > > A crash would happen which had memory corruption involved. But the > above seemed always to be in play. > > Now, I changed the logic from disabling context level recursion to > disabling recursion at all. That is, the original code had used a > series of bits to test for recursion (via helper functions) that would > allow for the callback to be preempted by an interrupt, and then be > called again. > > The new code sets a per_cpu flag, and will not allow the callback to > recurse if it were preempted by an interrupt. No real need to allow for > that anyway. > > I can go and debug this further, because I'm nervous that lockdep may > have some kind of bug that the function tracer can trigger. But I'm not > too concerned about it. > > Here's the diff of the new code against the previous code. > > Paul, can I still keep all your acks and reviewed-bys on this?
Yep, but some questions below.
Thanx, Paul
> -- Steve > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index 310b727..899c8c1 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -1373,7 +1373,7 @@ ftrace_hash_rec_enable(struct ftrace_ops *ops, int filter_hash); > > static int ftrace_convert_size_to_bits(int size) > { > - int bits; > + int bits = 0; > > /* > * Make the hash size about 1/2 the # found > diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c > index f5a9031..0883069 100644 > --- a/kernel/trace/trace_functions.c > +++ b/kernel/trace/trace_functions.c > @@ -561,16 +561,21 @@ static inline int init_func_cmd_traceon(void) > > #ifdef CONFIG_FTRACE_UNSAFE_RCU_CHECKER > > +static DEFINE_PER_CPU(int, ftrace_rcu_running); > static atomic_t ftrace_unsafe_rcu_disabled; > > void ftrace_unsafe_rcu_checker_disable(void) > { > atomic_inc(&ftrace_unsafe_rcu_disabled); > + /* Make sure the update is seen immediately */ > + smp_wmb();
smp_mb__after_atomic_inc()?
> } > > void ftrace_unsafe_rcu_checker_enable(void) > { > atomic_dec(&ftrace_unsafe_rcu_disabled); > + /* Make sure the update is seen immediately */ > + smp_wmb();
smp_mb__after_atomic_dec()?
> } > > static DEFINE_PER_CPU(unsigned long, ftrace_rcu_func); > @@ -588,15 +593,14 @@ static void > ftrace_unsafe_callback(unsigned long ip, unsigned long parent_ip, > struct ftrace_ops *op, struct pt_regs *pt_regs) > { > - int bit; > - > + /* Make sure we see disabled or not first */ > + smp_rmb();
smp_mb__before_atomic_inc()?
> if (atomic_read(&ftrace_unsafe_rcu_disabled)) > return; > > preempt_disable_notrace(); > > - bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX); > - if (bit < 0) > + if (this_cpu_read(ftrace_rcu_running)) > goto out; > > if (WARN_ONCE(ftrace_rcu_unsafe(ip), > @@ -604,13 +608,15 @@ ftrace_unsafe_callback(unsigned long ip, unsigned long parent_ip, > (void *)ip)) > goto out; > > + this_cpu_write(ftrace_rcu_running, 1); > this_cpu_write(ftrace_rcu_func, ip); > + > /* Should trigger a RCU splat if called from unsafe RCU function */ > rcu_read_lock(); > rcu_read_unlock(); > this_cpu_write(ftrace_rcu_func, 0); > > - trace_clear_recursion(bit); > + this_cpu_write(ftrace_rcu_running, 0); > out: > preempt_enable_notrace(); > } >
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