On Wed, Aug 23, 2017 at 01:58:47PM +0200, Peter Zijlstra wrote:
> 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,
I'm not sure if this solve the issue perfectly, but anyway it should be
a recursive version after fixing lockdep, regardless of the issue.
> but that would mean hitting a lockdep limitation on recursive locks.
Fo now, work-around might be needed.
> Also, unconditinoally switching to recursive-read here would fail to
> detect the actual deadlock on single-threaded workqueues, which do
Do you mean it's true even in case having fixed lockdep properly?
Could you explain why if so? IMHO, I don't think so.
> @@ -4751,15 +4751,31 @@ static inline void invalidate_xhlock(str
> * 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.
> + */
This very work-around code _must_ be removed after fixing read-recursive
thing in lockdep. I think it would be better to add a tag(comment)
saying it.
> + 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)
