Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb_writeback()
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2016-02-25 02:41:01
Also in:
linux-mm, lkml
On Wed, 24 Feb 2016 14:47:23 -0800 Yang Shi [off-list ref] wrote:
commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update writeback
tracepoints to report cgroup") made writeback tracepoints report cgroup
writeback, but it may trigger the below bug on -rt kernel due to the list_lock
held for the for loop in wb_writeback().list_lock is a sleeping mutex, it's not disabling preemption. Moving it doesn't make a difference.
BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930 in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3
Something else disabled preemption. And note, nothing in the tracepoint should have called a sleeping function.
INFO: lockdep is turned off. Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830 CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20 Hardware name: Freescale Layerscape 2085a RDB Board (DT) Workqueue: writeback wb_workfn (flush-7:0) Call trace: [<ffffffc00008d708>] dump_backtrace+0x0/0x200 [<ffffffc00008d92c>] show_stack+0x24/0x30 [<ffffffc0007b0f40>] dump_stack+0x88/0xa8 [<ffffffc000127d74>] ___might_sleep+0x2ec/0x300 [<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8 [<ffffffc0003e0548>] kernfs_path_len+0x30/0x90 [<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8
How accurate is this trace back? Here's the code that is executed in this tracepoint: TP_fast_assign( struct device *dev = bdi->dev; if (!dev) dev = default_backing_dev_info.dev; strncpy(__entry->name, dev_name(dev), 32); __entry->nr_pages = work->nr_pages; __entry->sb_dev = work->sb ? work->sb->s_dev : 0; __entry->sync_mode = work->sync_mode; __entry->for_kupdate = work->for_kupdate; __entry->range_cyclic = work->range_cyclic; __entry->for_background = work->for_background; __entry->reason = work->reason; ), See anything that would sleep?
[<ffffffc000374f90>] wb_writeback+0x620/0x830
[<ffffffc000376224>] wb_workfn+0x61c/0x950
[<ffffffc000110adc>] process_one_work+0x3ac/0xb30
[<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
[<ffffffc00011a9e8>] kthread+0x190/0x1b0
[<ffffffc000086ca0>] ret_from_fork+0x10/0x30
The list_lock was moved outside the for loop by commit
e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io()
into wb_writeback())", however, the commit log says "No behavior change", so
it sounds safe to have the list_lock acquired inside the for loop as it did
before.
Just acquire list_lock at the necessary points and keep all writeback
tracepoints outside the critical area protected by list_lock in
wb_writeback().But list_lock itself is a sleeping lock. This doesn't make sense. This is not the bug you are looking for. -- Steve
quoted hunk ↗ jump to hunk
Signed-off-by: Yang Shi <redacted> --- fs/fs-writeback.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 1f76d89..9b7b5f6 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c@@ -1623,7 +1623,6 @@ static long wb_writeback(struct bdi_writeback *wb, work->older_than_this = &oldest_jif; blk_start_plug(&plug); - spin_lock(&wb->list_lock); for (;;) { /* * Stop writeback when nr_pages has been consumed@@ -1661,15 +1660,19 @@ static long wb_writeback(struct bdi_writeback *wb, oldest_jif = jiffies; trace_writeback_start(wb, work); + + spin_lock(&wb->list_lock); if (list_empty(&wb->b_io)) queue_io(wb, work); if (work->sb) progress = writeback_sb_inodes(work->sb, wb, work); else progress = __writeback_inodes_wb(wb, work); - trace_writeback_written(wb, work); wb_update_bandwidth(wb, wb_start); + spin_unlock(&wb->list_lock); + + trace_writeback_written(wb, work); /* * Did we write something? Try for more@@ -1693,15 +1696,14 @@ static long wb_writeback(struct bdi_writeback *wb, */ if (!list_empty(&wb->b_more_io)) { trace_writeback_wait(wb, work); + spin_lock(&wb->list_lock); inode = wb_inode(wb->b_more_io.prev); - spin_lock(&inode->i_lock); spin_unlock(&wb->list_lock); + spin_lock(&inode->i_lock); /* This function drops i_lock... */ inode_sleep_on_writeback(inode); - spin_lock(&wb->list_lock); } } - spin_unlock(&wb->list_lock); blk_finish_plug(&plug); return nr_pages - work->nr_pages;
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