From 9e0af23764344f7f1b68e4eefbe7dc865018b63d Mon Sep 17 00:00:00 2001 From: Liu Bo Date: Fri, 15 Aug 2014 23:36:53 +0800 Subject: Btrfs: fix task hang under heavy compressed write This has been reported and discussed for a long time, and this hang occurs in both 3.15 and 3.16. Btrfs now migrates to use kernel workqueue, but it introduces this hang problem. Btrfs has a kind of work queued as an ordered way, which means that its ordered_func() must be processed in the way of FIFO, so it usually looks like -- normal_work_helper(arg) work = container_of(arg, struct btrfs_work, normal_work); work->func() <---- (we name it work X) for ordered_work in wq->ordered_list ordered_work->ordered_func() ordered_work->ordered_free() The hang is a rare case, first when we find free space, we get an uncached block group, then we go to read its free space cache inode for free space information, so it will file a readahead request btrfs_readpages() for page that is not in page cache __do_readpage() submit_extent_page() btrfs_submit_bio_hook() btrfs_bio_wq_end_io() submit_bio() end_workqueue_bio() <--(ret by the 1st endio) queue a work(named work Y) for the 2nd also the real endio() So the hang occurs when work Y's work_struct and work X's work_struct happens to share the same address. A bit more explanation, A,B,C -- struct btrfs_work arg -- struct work_struct kthread: worker_thread() pick up a work_struct from @worklist process_one_work(arg) worker->current_work = arg; <-- arg is A->normal_work worker->current_func(arg) normal_work_helper(arg) A = container_of(arg, struct btrfs_work, normal_work); A->func() A->ordered_func() A->ordered_free() <-- A gets freed B->ordered_func() submit_compressed_extents() find_free_extent() load_free_space_inode() ... <-- (the above readhead stack) end_workqueue_bio() btrfs_queue_work(work C) B->ordered_free() As if work A has a high priority in wq->ordered_list and there are more ordered works queued after it, such as B->ordered_func(), its memory could have been freed before normal_work_helper() returns, which means that kernel workqueue code worker_thread() still has worker->current_work pointer to be work A->normal_work's, ie. arg's address. Meanwhile, work C is allocated after work A is freed, work C->normal_work and work A->normal_work are likely to share the same address(I confirmed this with ftrace output, so I'm not just guessing, it's rare though). When another kthread picks up work C->normal_work to process, and finds our kthread is processing it(see find_worker_executing_work()), it'll think work C as a collision and skip then, which ends up nobody processing work C. So the situation is that our kthread is waiting forever on work C. Besides, there're other cases that can lead to deadlock, but the real problem is that all btrfs workqueue shares one work->func, -- normal_work_helper, so this makes each workqueue to have its own helper function, but only a wraper pf normal_work_helper. With this patch, I no long hit the above hang. Signed-off-by: Liu Bo Signed-off-by: Chris Mason --- fs/btrfs/async-thread.h | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-) (limited to 'fs/btrfs/async-thread.h') diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h index 9c6b66d1..e9e31c9 100644 --- a/fs/btrfs/async-thread.h +++ b/fs/btrfs/async-thread.h @@ -19,12 +19,14 @@ #ifndef __BTRFS_ASYNC_THREAD_ #define __BTRFS_ASYNC_THREAD_ +#include struct btrfs_workqueue; /* Internal use only */ struct __btrfs_workqueue; struct btrfs_work; typedef void (*btrfs_func_t)(struct btrfs_work *arg); +typedef void (*btrfs_work_func_t)(struct work_struct *arg); struct btrfs_work { btrfs_func_t func; @@ -38,11 +40,35 @@ struct btrfs_work { unsigned long flags; }; +#define BTRFS_WORK_HELPER_PROTO(name) \ +void btrfs_##name(struct work_struct *arg) + +BTRFS_WORK_HELPER_PROTO(worker_helper); +BTRFS_WORK_HELPER_PROTO(delalloc_helper); +BTRFS_WORK_HELPER_PROTO(flush_delalloc_helper); +BTRFS_WORK_HELPER_PROTO(cache_helper); +BTRFS_WORK_HELPER_PROTO(submit_helper); +BTRFS_WORK_HELPER_PROTO(fixup_helper); +BTRFS_WORK_HELPER_PROTO(endio_helper); +BTRFS_WORK_HELPER_PROTO(endio_meta_helper); +BTRFS_WORK_HELPER_PROTO(endio_meta_write_helper); +BTRFS_WORK_HELPER_PROTO(endio_raid56_helper); +BTRFS_WORK_HELPER_PROTO(rmw_helper); +BTRFS_WORK_HELPER_PROTO(endio_write_helper); +BTRFS_WORK_HELPER_PROTO(freespace_write_helper); +BTRFS_WORK_HELPER_PROTO(delayed_meta_helper); +BTRFS_WORK_HELPER_PROTO(readahead_helper); +BTRFS_WORK_HELPER_PROTO(qgroup_rescan_helper); +BTRFS_WORK_HELPER_PROTO(extent_refs_helper); +BTRFS_WORK_HELPER_PROTO(scrub_helper); +BTRFS_WORK_HELPER_PROTO(scrubwrc_helper); +BTRFS_WORK_HELPER_PROTO(scrubnc_helper); + struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name, int flags, int max_active, int thresh); -void btrfs_init_work(struct btrfs_work *work, +void btrfs_init_work(struct btrfs_work *work, btrfs_work_func_t helper, btrfs_func_t func, btrfs_func_t ordered_func, btrfs_func_t ordered_free); -- cgit v1.1