Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

Peter Zijlstra
2017-08-09T10:24:50Z

On Wed, Aug 09, 2017 at 11:51:07AM +0200, Peter Zijlstra wrote:
> On Mon, Aug 07, 2017 at 04:12:56PM +0900, Byungchul Park wrote:
> > +static inline void wait_for_completion(struct completion *x)
> > +{
> > + complete_acquire(x);
> > + __wait_for_completion(x);
> > + complete_release(x);
> > +}
> > +
> > +static inline void wait_for_completion_io(struct completion *x)
> > +{
> > + complete_acquire(x);
> > + __wait_for_completion_io(x);
> > + complete_release(x);
> > +}
> > +
> > +static inline int wait_for_completion_interruptible(struct completion *x)
> > +{
> > + int ret;
> > + complete_acquire(x);
> > + ret = __wait_for_completion_interruptible(x);
> > + complete_release(x);
> > + return ret;
> > +}
> > +
> > +static inline int wait_for_completion_killable(struct completion *x)
> > +{
> > + int ret;
> > + complete_acquire(x);
> > + ret = __wait_for_completion_killable(x);
> > + complete_release(x);
> > + return ret;
> > +}
>
> I don't understand, why not change __wait_for_common() ?

That is what is wrong with the below?

Yes, it adds acquire/release to the timeout variants too, but I don't
see why we should exclude those, and even if we'd want to do that, it
would be trivial:

bool timo = (timeout == MAX_SCHEDULE_TIMEOUT);

if (!timo)
complete_acquire(x);

/* ... */

if (!timo)
complete_release(x);

But like said, I think we very much want to annotate waits with timeouts
too. Hitting the max timo doesn't necessarily mean we'll make fwd
progress, we could be stuck in a loop doing something else again before
returning to wait.

Also, even if we'd make fwd progress, hitting that max timo is still not
desirable.

---
Subject: lockdep: Apply crossrelease to completions
From: Byungchul Park <byungchul.park@lge.com>
Date: Mon, 7 Aug 2017 16:12:56 +0900

Although wait_for_completion() and its family can cause deadlock, the
lock correctness validator could not be applied to them until now,
because things like complete() are usually called in a different context
from the waiting context, which violates lockdep's assumption.

Thanks to CONFIG_LOCKDEP_CROSSRELEASE, we can now apply the lockdep
detector to those completion operations. Applied it.

Cc: npiggin@gmail.com
Cc: mingo@kernel.org
Cc: akpm@linux-foundation.org
Cc: tglx@linutronix.de
Cc: boqun.feng@gmail.com
Cc: willy@infradead.org
Cc: walken@google.com
Cc: kernel-team@lge.com
Cc: kirill@shutemov.name
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/completion.h | 45 ++++++++++++++++++++++++++++++++++++++++++++-
kernel/sched/completion.c | 11 +++++++++++
lib/Kconfig.debug | 8 ++++++++
3 files changed, 63 insertions(+), 1 deletion(-)

--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -9,6 +9,9 @@
*/

#include <linux/wait.h>
+#ifdef CONFIG_LOCKDEP_COMPLETE
+#include <linux/lockdep.h>
+#endif

/*
* struct completion - structure used to maintain state for a "completion"
@@ -25,10 +28,50 @@
struct completion {
unsigned int done;
wait_queue_head_t wait;
+#ifdef CONFIG_LOCKDEP_COMPLETE
+ struct lockdep_map_cross map;
+#endif
};

+#ifdef CONFIG_LOCKDEP_COMPLETE
+static inline void complete_acquire(struct completion *x)
+{
+ lock_acquire_exclusive((struct lockdep_map *)&x->map, 0, 0, NULL, _RET_IP_);
+}
+
+static inline void complete_release(struct completion *x)
+{
+ lock_release((struct lockdep_map *)&x->map, 0, _RET_IP_);
+}
+
+static inline void complete_release_commit(struct completion *x)
+{
+ lock_commit_crosslock((struct lockdep_map *)&x->map);
+}
+
+#define init_completion(x) \
+do { \
+ static struct lock_class_key __key; \
+ lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map, \
+ "(complete)" #x, \
+ &__key, 0); \
+ __init_completion(x); \
+} while (0)
+#else
+#define init_completion(x) __init_completion(x)
+static inline void complete_acquire(struct completion *x) {}
+static inline void complete_release(struct completion *x) {}
+static inline void complete_release_commit(struct completion *x) {}
+#endif
+
+#ifdef CONFIG_LOCKDEP_COMPLETE
+#define COMPLETION_INITIALIZER(work) \
+ { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait), \
+ STATIC_CROSS_LOCKDEP_MAP_INIT("(complete)" #work, &(work)) }
+#else
#define COMPLETION_INITIALIZER(work) \
{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
+#endif

#define COMPLETION_INITIALIZER_ONSTACK(work) \
({ init_completion(&work); work; })
@@ -70,7 +113,7 @@ struct completion {
* This inline function will initialize a dynamically created completion
* structure.
*/
-static inline void init_completion(struct completion *x)
+static inline void __init_completion(struct completion *x)
{
x->done = 0;
init_waitqueue_head(&x->wait);
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -32,6 +32,12 @@ void complete(struct completion *x)
unsigned long flags;

spin_lock_irqsave(&x->wait.lock, flags);
+
+ /*
+ * Perform commit of crossrelease here.
+ */
+ complete_release_commit(x);
+
if (x->done != UINT_MAX)
x->done++;
__wake_up_locked(&x->wait, TASK_NORMAL, 1);
@@ -92,9 +98,14 @@ __wait_for_common(struct completion *x,
{
might_sleep();

+ complete_acquire(x);
+
spin_lock_irq(&x->wait.lock);
timeout = do_wait_for_common(x, action, timeout, state);
spin_unlock_irq(&x->wait.lock);
+
+ complete_release(x);
+
return timeout;
}

--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1085,6 +1085,14 @@ config LOCKDEP_CROSSRELEASE
such as page locks or completions can use the lock correctness
detector, lockdep.

+config LOCKDEP_COMPLETE
+ bool "Lock debugging: allow completions to use deadlock detector"
+ select LOCKDEP_CROSSRELEASE
+ default n
+ help
+ A deadlock caused by wait_for_completion() and complete() can be
+ detected by lockdep using crossrelease feature.
+
config PROVE_LOCKING
bool "Lock debugging: prove locking correctness"
depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT