On Thu, 25 Feb 2016 15:16:54 -0800
"Shi, Yang" <yang.shi@xxxxxxxxxx> 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
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