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SubjectRe: [PATCH v9 5/7] tracing: Centralize preemptirq tracepoints and unify their usage
On Thu, Jun 28, 2018 at 11:21:47AM -0700, Joel Fernandes wrote:
> One note, I have to check for lockdep recursion in the code that calls
> the trace events API and bail out if we're in lockdep recursion

I'm not seeing any new lockdep_recursion checks...

> protection to prevent something like the following case: a spin_lock is
> taken. Then lockdep_acquired is called. That does a raw_local_irq_save
> and then sets lockdep_recursion, and then calls __lockdep_acquired. In
> this function, a call to get_lock_stats happens which calls
> preempt_disable, which calls trace IRQS off somewhere which enters my
> tracepoint code and sets the tracing_irq_cpu flag to prevent recursion.
> This flag is then never cleared causing lockdep paths to never be
> entered and thus causing splats and other bad things.

Would it not be much easier to avoid that entirely, afaict all
get/put_lock_stats() callers already have IRQs disabled, so that
(traced) preempt fiddling is entirely superfluous.

---
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 5fa4d3138bf1..8f5ce0048d15 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -248,12 +248,7 @@ void clear_lock_stats(struct lock_class *class)

static struct lock_class_stats *get_lock_stats(struct lock_class *class)
{
- return &get_cpu_var(cpu_lock_stats)[class - lock_classes];
-}
-
-static void put_lock_stats(struct lock_class_stats *stats)
-{
- put_cpu_var(cpu_lock_stats);
+ return &this_cpu_ptr(&cpu_lock_stats)[class - lock_classes];
}

static void lock_release_holdtime(struct held_lock *hlock)
@@ -271,7 +266,6 @@ static void lock_release_holdtime(struct held_lock *hlock)
lock_time_inc(&stats->read_holdtime, holdtime);
else
lock_time_inc(&stats->write_holdtime, holdtime);
- put_lock_stats(stats);
}
#else
static inline void lock_release_holdtime(struct held_lock *hlock)
@@ -4090,7 +4084,6 @@ __lock_contended(struct lockdep_map *lock, unsigned long ip)
stats->contending_point[contending_point]++;
if (lock->cpu != smp_processor_id())
stats->bounces[bounce_contended + !!hlock->read]++;
- put_lock_stats(stats);
}

static void
@@ -4138,7 +4131,6 @@ __lock_acquired(struct lockdep_map *lock, unsigned long ip)
}
if (lock->cpu != cpu)
stats->bounces[bounce_acquired + !!hlock->read]++;
- put_lock_stats(stats);

lock->cpu = cpu;
lock->ip = ip;
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