On Thu, Jan 19, 2012 at 01:39:21AM +0530, Rabin Vincent wrote: > On Tue, Jan 17, 2012 at 09:02, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > > On Sun, Jan 15, 2012 at 08:58:06PM +0530, Rabin Vincent wrote: > >> Unable to handle kernel NULL pointer dereference at virtual address 0000002c > >> pgd = c0004000 > >> [0000002c] *pgd=00000000 > >> Internal error: Oops: 17 [#1] PREEMPT SMP > >> PC is at ftrace_raw_event_writeback_single_inode_template+0x60/0xe4 > >> LR is at ftrace_raw_event_writeback_single_inode_template+0x50/0xe4 > >> > >> The full trace+log is attached. My kernel (current linus) has a delay > >> inserted in __mark_inode_dirty, to easily trigger the condition: > > > > Rabin, thanks for showing the helpful details! It should be fixable by > > the use of inode_to_bdi(): > > Thanks, this fixes that one. > > However, I've found one more race condition leading to a crash when > tracing is enabled, this time from the writeback:queue trace point from > bdi_queue_work(). The cause is the same, i.e. bdi->dev is NULL. This > was produced with the help of the following delay patch. trace+log is > attached. Rabin, this should fix the bug. Note that I take no efforts to remove the to-be-queued and already-queued works. I'm also a bit afraid if the traces in the balance_dirty_pages() path (trace_balance_dirty_pages, trace_bdi_dirty_ratelimit and writeback_wake_background) will have similar NULL dereference bug. Do you test it by physically hot removing a SD card, or with some detach command or emulation? Thanks, Fengguang --- Subject: writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue Date: Sat Feb 04 20:54:03 CST 2012 When the SD card is hot removed without umount, del_gendisk() will call bdi_unregister() but not destroy/free it. This leaves the bdi in the bdi->dev = NULL, bdi->wb.task = NULL, bdi->bdi_list removed state. If someone gets the bdi before bdi_unregister() and calls bdi_queue_work() after the unregister, trace_writeback_queue will be dereferencing the NULL bdi->dev. Fix it with a simple test for NULL. LKML-reference: http://lkml.org/lkml/2012/1/18/346 Reported-by: Rabin Vincent <rabin@xxxxxx> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> --- include/trace/events/writeback.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) --- linux-next.orig/include/trace/events/writeback.h 2012-02-04 20:51:01.000000000 +0800 +++ linux-next/include/trace/events/writeback.h 2012-02-04 20:54:00.000000000 +0800 @@ -47,7 +47,10 @@ DECLARE_EVENT_CLASS(writeback_work_class __field(int, reason) ), TP_fast_assign( - strncpy(__entry->name, dev_name(bdi->dev), 32); + 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; -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
