On Thu, 25 Feb 2016 15:16:54 -0800 "Shi, Yang" yang.shi@linaro.org wrote:
Actually, regardless whether this is the right fix for the splat, it makes me be wondering if the spin lock which protects the whole for loop is really necessary. It sounds feasible to move it into the for loop and just protect the necessary area.
That's a separate issue, which may have its own merits that should be decided by the writeback folks.
INFO: lockdep is turned off. Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
Can you disassemble the vmlinux file to see exactly where that call is. I use gdb to find the right locations. gdb> li *0xffffffc000374a5c gdb> disass 0xffffffc000374a5c
I use gdb to get the code too.
It does point to the spin_lock.
(gdb) list *0xffffffc000374a5c 0xffffffc000374a5c is in wb_writeback (fs/fs-writeback.c:1621). 1616 1617 oldest_jif = jiffies; 1618 work->older_than_this = &oldest_jif; 1619 1620 blk_start_plug(&plug); 1621 spin_lock(&wb->list_lock); 1622 for (;;) { 1623 /* 1624 * Stop writeback when nr_pages has been consumed 1625 */
The disassemble: 0xffffffc000374a58 <+232>: bl 0xffffffc0001300b0
The above is the place it recorded. But I just realized, this isn't the issue. I know where the problem is.
<migrate_disable> 0xffffffc000374a5c <+236>: mov x0, x22 0xffffffc000374a60 <+240>: bl 0xffffffc000d5d518 <rt_spin_lock>
DECLARE_EVENT_CLASS(writeback_work_class, TP_PROTO(struct bdi_writeback *wb, struct wb_writeback_work *work), TP_ARGS(wb, work), TP_STRUCT__entry( __array(char, name, 32) __field(long, nr_pages) __field(dev_t, sb_dev) __field(int, sync_mode) __field(int, for_kupdate) __field(int, range_cyclic) __field(int, for_background) __field(int, reason) __dynamic_array(char, cgroup, __trace_wb_cgroup_size(wb))
Ah, thanks for pointing that out. I missed that.
It sounds not correct if tracepoint doesn't allow sleep.
I considered to change sleeping lock to raw lock in kernfs_* functions, but it sounds not reasonable since they are used heavily by cgroup.
It is the kernfs_* that can't sleep. Tracepoints use rcu_read_lock_sched_notrace(), which disables preemption, and not only that, hides itself from lockdep as the last place to disable preemption.
Is there a way to not use the kernfs_* function? At least for -rt?
-- Steve