On Wed, 24 Feb 2016 14:47:23 -0800 Yang Shi <yang.shi@xxxxxxxxxx> 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 > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx> > --- > 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; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>