With CROSSRELEASE feature introduced, such a potential deadlock is reported:
> Worker A : acquired of wfc.work -> wait for cpu_hotplug_lock to be released > Task B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released > Task C : acquired of lock#3 -> wait for completion of barr->done > (Task C is in lru_add_drain_all_cpuslocked()) > Worker D : wait for wfc.work to be released -> will complete barr->done However, given this very case, such a dead lock could not happen because Task C's barr->done and Worker D's barr->done can not be the same instance. So this is a false positive. The reason of this false positive is we initialize all wq_barrier::done at insert_wq_barrier() via init_completion(), which makes them belong to the same lock class, therefore, impossible circles are reported. To fix this, explicitly initialize the lockdep map for wq_barrier::done in insert_wq_barrier(), so that the lock class key of wq_barrier::done is a subkey of the corresponding work_struct, as a result we won't build a dependency between a wq_barrier with a unrelated work, and we can differ wq barriers based on the related works, so the false positive above is avoided. Also define the empty lockdep_init_map_crosslock() for !CROSSRELEASE to make the code simple and away from unnecessary #ifdefs. Reported-by: Ingo Molnar <mi...@kernel.org> Signed-off-by: Boqun Feng <boqun.f...@gmail.com> Cc: Byungchul Park <byungchul.p...@lge.com> --- include/linux/lockdep.h | 1 + kernel/workqueue.c | 11 ++++++++++- 2 files changed, 11 insertions(+), 1 deletion(-) diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h index 651cc61af041..fc827cab6d6e 100644 --- a/include/linux/lockdep.h +++ b/include/linux/lockdep.h @@ -583,6 +583,7 @@ extern void crossrelease_hist_end(enum xhlock_context_t c); extern void lockdep_init_task(struct task_struct *task); extern void lockdep_free_task(struct task_struct *task); #else +#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0) /* * To initialize a lockdep_map statically use this macro. * Note that _name must not be NULL. diff --git a/kernel/workqueue.c b/kernel/workqueue.c index e86733a8b344..861301c2b638 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -2476,7 +2476,16 @@ static void insert_wq_barrier(struct pool_workqueue *pwq, */ INIT_WORK_ONSTACK(&barr->work, wq_barrier_func); __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work)); - init_completion(&barr->done); + + /* + * Explicitly init the crosslock for wq_barrier::done, make its lock + * key is a subkey of the corresponding work. As a result we won't + * build a dependency between a wq_barrier::done and a unrelated work. + */ + lockdep_init_map_crosslock((struct lockdep_map *)&barr->done.map, + "(complete)wq_barr::done", + target->lockdep_map.key, 1); + __init_completion(&barr->done); barr->task = current; /* -- 2.14.1