Re: [PATCHv7 3/8] printk: introduce per-cpu safe_print seq buffer

[Steven Rostedt]

On Tue, 27 Dec 2016 23:16:06 +0900
Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> wrote:

> This patch extends the idea of NMI per-cpu buffers to regions
> that may cause recursive printk() calls and possible deadlocks.
> Namely, printk() can't handle printk calls from schedule code
> or printk() calls from lock debugging code (spin_dump() for instance);
> because those may be called with `sem->lock' already taken or any
> other `critical' locks (p->pi_lock, etc.). An example of deadlock
> can be
> 
>  vprintk_emit()
>   console_unlock()
>    up()                        << raw_spin_lock_irqsave(&sem->lock, flags);
>     wake_up_process()
>      try_to_wake_up()
>       ttwu_queue()
>        ttwu_activate()
>         activate_task()
>          enqueue_task()
>           enqueue_task_fair()
>            cfs_rq_of()
>             task_of()
>              WARN_ON_ONCE(!entity_is_task(se))
>               vprintk_emit()
>                console_trylock()
>                 down_trylock()
>                  raw_spin_lock_irqsave(&sem->lock, flags)
>                  ^^^^ deadlock
> 
> and some other cases.
> 
> Just like in NMI implementation, the solution uses a per-cpu
> `printk_func' pointer to 'redirect' printk() calls to a 'safe'
> callback, that store messages in a per-cpu buffer and flushes
> them back to logbuf buffer later.
> 
> Usage example:
> 
>  printk()
>   printk_safe_enter_irqsave(flags)
>   //
>   //  any printk() call from here will endup in vprintk_safe(),
>   //  that stores messages in a special per-CPU buffer.
>   //
>   printk_safe_exit_irqrestore(flags)
> 
> The 'redirection' mechanism, though, has been reworked, as suggested
> by Petr Mladek. Instead of using a per-cpu @print_func callback we now
> keep a per-cpu printk-context variable and call either default or nmi
> vprintk function depending on its value. printk_nmi_entrer/exit and
> printk_safe_enter/exit, thus, just set/celar corresponding bits in
> printk-context functions.
> 
> The patch only adds printk_safe support, we don't use it yet.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>


/me does the actual reviewed-by now :-p

Yes, I like this approach. I probably would have done it pretty much
the same way.

Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

-- Steve