Re: [PATCHv7 6/8] printk: use printk_safe buffers in printk

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (02/02/17 16:20), Petr Mladek wrote:
> > well, I wouldn't say that printk_deferred() has less chances. I see your
> > point, of course. but with printk_deferred() we, at least, will have messages
> > in logbuf (or printk_safe buffers), so they can appear in crash dump, for
> > instance. that "later" part can be sysrq, for example, or panic->flush_on_panic(),
> > etc. if "normal" printk->queue irq_work doesn't work.
> > 
> > needless to say, that in this particular case (WARN from sched), if the
> > first printk() out of N printk()-s, which sched core calls to dump_stack(),
> > deadlocks, then we got nothing to print/dump.
> 
> An always deferred printk() or another deferred ways are future work.
> We should try to find a good solution, definitely.
> 
> The question is what to do with this patch. We need to change things
> step by step. The printk_safe patchset is one of them and looks
> almost ready.
> 
> The lockdep warnings are correct and help to find locations where
> scheduler warnings might cause a deadlock.

I like that lockdep warning. and looking at it... I think lockdep does
not add any additional risks.

we are in deadlock risky sched->printk condition due to WARN from
sched, not lockdep. the lockdep warning that we see happens after
we switch to printk_safe mode.


please see console_trylock()->__down_trylock_console_sem()


static int __down_trylock_console_sem(unsigned long ip)
{
...
 224        printk_safe_enter_irqsave(flags);
 225        lock_failed = down_trylock(&console_sem);   << print_circular_bug() comes from here
 226        printk_safe_exit_irqrestore(flags);
...
}

so the unsafe/safe printk 'map' should be as follows

[   13.090679] Call Trace:
[   13.090680]  dump_stack+0x86/0xc3
[   13.090680]  print_circular_bug+0x1be/0x210          << still in printk_safe
[   13.090680]  __lock_acquire+0x10e5/0x1270
[   13.090681]  lock_acquire+0xfd/0x200
[   13.090681]  ? down_trylock+0x14/0x40
[   13.090681]  _raw_spin_lock_irqsave+0x59/0x93
[   13.090681]  ? down_trylock+0x14/0x40
[   13.090682]  ? vprintk_emit+0x2c7/0x3a0
[   13.090682]  down_trylock+0x14/0x40
[   13.090682]  __down_trylock_console_sem+0x3c/0xc0    << we are in printk_safe now (!)
[   13.090683]  console_trylock+0x16/0x90
[   13.090683]  ? trace_hardirqs_off+0xd/0x10
[   13.090683]  vprintk_emit+0x2c7/0x3a0
[   13.090684]  ? update_load_avg+0x85b/0xb80
[   13.090684]  vprintk_default+0x29/0x50
[   13.090684]  vprintk_func+0x25/0x80                  << we are in unsafe printk here (!)
[   13.090684]  printk+0x52/0x6e
[   13.090685]  ? update_load_avg+0x85b/0xb80
[   13.090685]  __warn+0x39/0xf0
[   13.090685]  warn_slowpath_fmt+0x5f/0x80
[   13.090686]  update_load_avg+0x85b/0xb80
[   13.090686]  ? debug_smp_processor_id+0x17/0x20
[   13.090686]  detach_task_cfs_rq+0x3f/0x210
[   13.090687]  task_change_group_fair+0x24/0x100
[   13.090687]  sched_change_group+0x5f/0x110
[   13.090687]  sched_move_task+0x53/0x160
[   13.090687]  cpu_cgroup_attach+0x36/0x70
[   13.090688]  cgroup_migrate_execute+0x230/0x3f0
[   13.090688]  cgroup_migrate+0xce/0x140
[   13.090688]  ? cgroup_migrate+0x5/0x140
[   13.090689]  cgroup_attach_task+0x27f/0x3e0
[   13.090689]  ? cgroup_attach_task+0x9b/0x3e0
[   13.090689]  __cgroup_procs_write+0x30e/0x510
[   13.090690]  ? __cgroup_procs_write+0x70/0x510
[   13.090690]  cgroup_procs_write+0x14/0x20
[   13.090690]  cgroup_file_write+0x44/0x1e0
[   13.090690]  kernfs_fop_write+0x13c/0x1c0
[   13.090691]  __vfs_write+0x37/0x160
[   13.090691]  ? rcu_read_lock_sched_held+0x4a/0x80
[   13.090691]  ? rcu_sync_lockdep_assert+0x2f/0x60
[   13.090692]  ? __sb_start_write+0x10d/0x220
[   13.090692]  ? vfs_write+0x19b/0x1f0
[   13.090692]  ? security_file_permission+0x3b/0xc0
[   13.090693]  vfs_write+0xcb/0x1f0
[   13.090693]  SyS_write+0x58/0xc0
[   13.090693]  entry_SYSCALL_64_fastpath+0x1f/0xc2


that unsafe console_trylock() is not caused by lockdep. yes, we can
deadlock in down_trylock(), but lockdep is not the root cause. and
if we will disable lockdep, sched->printk->console_trylock() still
will have pretty much same chances to deadlock.

let me know if I'm missing something.


> One solution would be to keep lockdep as is in this patch. It means
> to hide existing risk until we have some reasonable printk_deferred()
> solution.

well, yes. this is still a possible way to go (until the deferred printk()).


> Another solution would to keep this patch as is and implement
> WARN*_DEFERRED() variants that would either use
> printk_safe_enter()/exit() as the currently usable deferred and
> lockless solution. Or they could just disable lockdep and hide
> the report for now. These deferred variants should be
> used on all locations reported by lockdep where we want to accept the
> risk. We will at least know where the potential risk is and could find
> a proper solution later.

WARN*_DEFERRED() looks to me like almost unmaintainable thing.
too much work; a never ending work.


> Note that I do not like hiding problems but they were hidden before this
> patchset as well. I am just looking for the best way forward.

sure.

	-ss