summaryrefslogtreecommitdiff
path: root/kernel
diff options
context:
space:
mode:
authorPeter Zijlstra <peterz@infradead.org>2017-08-23 13:23:30 +0200
committerIngo Molnar <mingo@kernel.org>2017-08-25 11:06:33 +0200
commite6f3faa734a00c606b7b06c6b9f15e5627d3245b (patch)
tree4c3f0047d1fa1796442512e03147c40c026f25a8 /kernel
parenta1d14934ea4b9db816a8dbfeab1c3e7204a0d871 (diff)
locking/lockdep: Fix workqueue crossrelease annotation
The new completion/crossrelease annotations interact unfavourable with the extant flush_work()/flush_workqueue() annotations. The problem is that when a single work class does: wait_for_completion(&C) and complete(&C) in different executions, we'll build dependencies like: lock_map_acquire(W) complete_acquire(C) and lock_map_acquire(W) complete_release(C) which results in the dependency chain: W->C->W, which lockdep thinks spells deadlock, even though there is no deadlock potential since works are ran concurrently. One possibility would be to change the work 'lock' to recursive-read, but that would mean hitting a lockdep limitation on recursive locks. Also, unconditinoally switching to recursive-read here would fail to detect the actual deadlock on single-threaded workqueues, which do have a problem with this. For now, forcefully disregard these locks for crossrelease. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Tejun Heo <tj@kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: boqun.feng@gmail.com Cc: byungchul.park@lge.com Cc: david@fromorbit.com Cc: johannes@sipsolutions.net Cc: oleg@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/locking/lockdep.c56
-rw-r--r--kernel/workqueue.c23
2 files changed, 58 insertions, 21 deletions
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 66011c9f5df3..f73ca595b81e 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4629,7 +4629,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
* the index to point to the last entry, which is already invalid.
*/
crossrelease_hist_end(XHLOCK_PROC);
- crossrelease_hist_start(XHLOCK_PROC);
+ crossrelease_hist_start(XHLOCK_PROC, false);
}
void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
@@ -4725,25 +4725,25 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
/*
* Lock history stacks; we have 3 nested lock history stacks:
*
- * Hard IRQ
- * Soft IRQ
- * History / Task
+ * 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.
+ * 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.
*
- * So what we do is rewind the history buffer and erase all our knowledge of
- * that temporal event.
- */
-
-/*
- * We need this to annotate lock history boundaries. 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 (in general).
- * Therefore we must not carry that (lock) dependency across works.
+ * The PROCess one is special though; it is used to annotate independence
+ * inside a task.
+ *
+ * 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
+ * (in general). Therefore we must not carry that (lock) dependency across
+ * works.
*
* This is true for many things; pretty much all kthreads fall into this
- * pattern, where they have an 'idle' state and future completions do not
+ * pattern, where they have an invariant state and future completions do not
* depend on past completions. Its just that since they all have the 'same'
* form -- the kthread does the same over and over -- it doesn't typically
* matter.
@@ -4751,15 +4751,31 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
* The same is true for system-calls, once a system call is completed (we've
* returned to userspace) the next system call does not depend on the lock
* history of the previous system call.
+ *
+ * They key property for independence, this invariant state, is that it must be
+ * a point where we hold no locks and have no history. Because if we were to
+ * hold locks, the restore at _end() would not necessarily recover it's history
+ * entry. Similarly, independence per-definition means it does not depend on
+ * prior state.
*/
-void crossrelease_hist_start(enum xhlock_context_t c)
+void crossrelease_hist_start(enum xhlock_context_t c, bool force)
{
struct task_struct *cur = current;
- if (cur->xhlocks) {
- cur->xhlock_idx_hist[c] = cur->xhlock_idx;
- cur->hist_id_save[c] = cur->hist_id;
+ if (!cur->xhlocks)
+ return;
+
+ /*
+ * We call this at an invariant point, no current state, no history.
+ */
+ 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)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8ad214dc15a9..c0331891dec1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2093,7 +2093,28 @@ __acquires(&pool->lock)
lock_map_acquire(&pwq->wq->lockdep_map);
lock_map_acquire(&lockdep_map);
- crossrelease_hist_start(XHLOCK_PROC);
+ /*
+ * Strictly speaking we should do start(PROC) without holding any
+ * locks, that is, before these two lock_map_acquire()'s.
+ *
+ * However, that would result in:
+ *
+ * A(W1)
+ * WFC(C)
+ * A(W1)
+ * C(C)
+ *
+ * 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
+ * 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);
trace_workqueue_execute_start(work);
worker->current_func(work);
/*