Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 25, 2017 at 10:11:14AM +0900, Byungchul Park wrote:
> I meant, this seems to be led from your mis-understanding of
> crossrelease_hist_{start, end}().

I have, several times now, explained why PROC is special.

You seem to still think it can be used like the soft/hard-irq ones, this
is fundamentally not so.

Does something like so help?

---
Subject: lockdep: Untangle xhlock history save/restore from task independence

Where XHLOCK_{SOFT,HARD} are save/restore points in the xhlocks[] to
ensure the temporal IRQ events don't interact with task state, the
XHLOCK_PROC is a fundament different beast that just happens to share
the interface.

The purpose of XHLOCK_PROC is to annotate independent execution inside
one task. For example workqueues, each work should appear to run in its
own 'pristine' 'task'.

Remove XHLOCK_PROC in favour of its own interface to avoid confusion.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/irqflags.h |  4 +--
 include/linux/lockdep.h  |  7 +++--
 kernel/locking/lockdep.c | 79 +++++++++++++++++++++++-------------------------
 kernel/workqueue.c       |  9 +++---
 4 files changed, 48 insertions(+), 51 deletions(-)

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 9bc050bc81b2..5fdd93bb9300 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -26,7 +26,7 @@
 # define trace_hardirq_enter()			\
 do {						\
 	current->hardirq_context++;		\
-	crossrelease_hist_start(XHLOCK_HARD, 0);\
+	crossrelease_hist_start(XHLOCK_HARD);	\
 } while (0)
 # define trace_hardirq_exit()			\
 do {						\
@@ -36,7 +36,7 @@ do {						\
 # define lockdep_softirq_enter()		\
 do {						\
 	current->softirq_context++;		\
-	crossrelease_hist_start(XHLOCK_SOFT, 0);\
+	crossrelease_hist_start(XHLOCK_SOFT);	\
 } while (0)
 # define lockdep_softirq_exit()			\
 do {						\
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 78bb7133abed..bfa8e0b0d6f1 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -551,7 +551,6 @@ struct pin_cookie { };
 enum xhlock_context_t {
 	XHLOCK_HARD,
 	XHLOCK_SOFT,
-	XHLOCK_PROC,
 	XHLOCK_CTX_NR,
 };
 
@@ -580,8 +579,9 @@ extern void lock_commit_crosslock(struct lockdep_map *lock);
 #define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
 	{ .name = (_name), .key = (void *)(_key), .cross = 0, }
 
-extern void crossrelease_hist_start(enum xhlock_context_t c, bool force);
+extern void crossrelease_hist_start(enum xhlock_context_t c);
 extern void crossrelease_hist_end(enum xhlock_context_t c);
+extern void lockdep_invariant_state(bool force);
 extern void lockdep_init_task(struct task_struct *task);
 extern void lockdep_free_task(struct task_struct *task);
 #else /* !CROSSRELEASE */
@@ -593,8 +593,9 @@ extern void lockdep_free_task(struct task_struct *task);
 #define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
 	{ .name = (_name), .key = (void *)(_key), }
 
-static inline void crossrelease_hist_start(enum xhlock_context_t c, bool force) {}
+static inline void crossrelease_hist_start(enum xhlock_context_t c) {}
 static inline void crossrelease_hist_end(enum xhlock_context_t c) {}
+static inline void lockdep_invariant_state(bool force) {}
 static inline void lockdep_init_task(struct task_struct *task) {}
 static inline void lockdep_free_task(struct task_struct *task) {}
 #endif /* CROSSRELEASE */
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index f73ca595b81e..44c8d0d17170 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4623,13 +4623,8 @@ asmlinkage __visible void lockdep_sys_exit(void)
 	/*
 	 * The lock history for each syscall should be independent. So wipe the
 	 * slate clean on return to userspace.
-	 *
-	 * crossrelease_hist_end() works well here even when getting here
-	 * without starting (i.e. just after forking), because it rolls back
-	 * the index to point to the last entry, which is already invalid.
 	 */
-	crossrelease_hist_end(XHLOCK_PROC);
-	crossrelease_hist_start(XHLOCK_PROC, false);
+	lockdep_invariant_state(false);
 }
 
 void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
@@ -4723,19 +4718,47 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
 }
 
 /*
- * Lock history stacks; we have 3 nested lock history stacks:
+ * Lock history stacks; we have 2 nested lock history stacks:
  *
  *   HARD(IRQ)
  *   SOFT(IRQ)
- *   PROC(ess)
  *
  * The thing is that once we complete a HARD/SOFT IRQ the future task locks
  * should not depend on any of the locks observed while running the IRQ.  So
  * what we do is rewind the history buffer and erase all our knowledge of that
  * temporal event.
- *
- * The PROCess one is special though; it is used to annotate independence
- * inside a task.
+ */
+
+void crossrelease_hist_start(enum xhlock_context_t c)
+{
+	struct task_struct *cur = current;
+
+	if (!cur->xhlocks)
+		return;
+
+	cur->xhlock_idx_hist[c] = cur->xhlock_idx;
+	cur->hist_id_save[c]    = cur->hist_id;
+}
+
+void crossrelease_hist_end(enum xhlock_context_t c)
+{
+	struct task_struct *cur = current;
+
+	if (cur->xhlocks) {
+		unsigned int idx = cur->xhlock_idx_hist[c];
+		struct hist_lock *h = &xhlock(idx);
+
+		cur->xhlock_idx = idx;
+
+		/* Check if the ring was overwritten. */
+		if (h->hist_id != cur->hist_id_save[c])
+			invalidate_xhlock(h);
+	}
+}
+
+/*
+ * lockdep_invariant_state() is used to annotate independence inside a task, to
+ * make one task look like multiple independent 'tasks'.
  *
  * Take for instance workqueues; each work is independent of the last. The
  * completion of a future work does not depend on the completion of a past work
@@ -4758,40 +4781,14 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
  * entry. Similarly, independence per-definition means it does not depend on
  * prior state.
  */
-void crossrelease_hist_start(enum xhlock_context_t c, bool force)
+void lockdep_invariant_state(bool force)
 {
-	struct task_struct *cur = current;
-
-	if (!cur->xhlocks)
-		return;
-
 	/*
 	 * We call this at an invariant point, no current state, no history.
+	 * Verify the former, enforce the latter.
 	 */
-	if (c == XHLOCK_PROC) {
-		/* verified the former, ensure the latter */
-		WARN_ON_ONCE(!force && cur->lockdep_depth);
-		invalidate_xhlock(&xhlock(cur->xhlock_idx));
-	}
-
-	cur->xhlock_idx_hist[c] = cur->xhlock_idx;
-	cur->hist_id_save[c]    = cur->hist_id;
-}
-
-void crossrelease_hist_end(enum xhlock_context_t c)
-{
-	struct task_struct *cur = current;
-
-	if (cur->xhlocks) {
-		unsigned int idx = cur->xhlock_idx_hist[c];
-		struct hist_lock *h = &xhlock(idx);
-
-		cur->xhlock_idx = idx;
-
-		/* Check if the ring was overwritten. */
-		if (h->hist_id != cur->hist_id_save[c])
-			invalidate_xhlock(h);
-	}
+	WARN_ON_ONCE(!force && current->lockdep_depth);
+	invalidate_xhlock(&xhlock(current->xhlock_idx));
 }
 
 static int cross_lock(struct lockdep_map *lock)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c0331891dec1..ab3c0dc8c7ed 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2094,8 +2094,8 @@ __acquires(&pool->lock)
 	lock_map_acquire(&pwq->wq->lockdep_map);
 	lock_map_acquire(&lockdep_map);
 	/*
-	 * Strictly speaking we should do start(PROC) without holding any
-	 * locks, that is, before these two lock_map_acquire()'s.
+	 * Strictly speaking we should mark the invariant state without holding
+	 * any locks, that is, before these two lock_map_acquire()'s.
 	 *
 	 * However, that would result in:
 	 *
@@ -2107,14 +2107,14 @@ __acquires(&pool->lock)
 	 * Which would create W1->C->W1 dependencies, even though there is no
 	 * actual deadlock possible. There are two solutions, using a
 	 * read-recursive acquire on the work(queue) 'locks', but this will then
-	 * hit the lockdep limitation on recursive locks, or simly discard
+	 * hit the lockdep limitation on recursive locks, or simply discard
 	 * these locks.
 	 *
 	 * AFAICT there is no possible deadlock scenario between the
 	 * flush_work() and complete() primitives (except for single-threaded
 	 * workqueues), so hiding them isn't a problem.
 	 */
-	crossrelease_hist_start(XHLOCK_PROC, true);
+	lockdep_invariant_state(true);
 	trace_workqueue_execute_start(work);
 	worker->current_func(work);
 	/*
@@ -2122,7 +2122,6 @@ __acquires(&pool->lock)
 	 * point will only record its address.
 	 */
 	trace_workqueue_execute_end(work);
-	crossrelease_hist_end(XHLOCK_PROC);
 	lock_map_release(&lockdep_map);
 	lock_map_release(&pwq->wq->lockdep_map);