linux-kernel - Re: [PATCHv6 6/7] printk: use printk

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Date:   Fri, 23 Dec 2016 10:53:41 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCHv6 6/7] printk: use printk_safe buffers in printk

On Fri 2016-12-23 10:46:43, Sergey Senozhatsky wrote:
> not every switch to printk_safe is "dictated" by logbuf_lock.
> down_trylock_console_sem(), for instance, takes semaphore spin_lock
> which already may be locked on the same CPU (*), so we need to be
> in safe mode:
> 
> vprintk_emit()
>  down_trylock()
>   raw_spin_lock_irqsave(&sem->lock, flags);
>   ...
>   raw_spin_unlock_irqrestore(&sem->lock, flags);
>    spin_dump()
>     printk()
>      vprintk_emit()
>       down_trylock()
>        raw_spin_lock_irqsave(&sem->lock, flags)   << deadlock
> 
> 
> and so on. IOW, "printk_save_enter()" != "logbuf_lock is acquired".

You are right. It seems that the

printk_safe_enter_irq()
printk_safe_exit_irq()

printk_safe_enter_irqsave(flags)
printk_safe_exit_irqrestore(flags)

variants make sense and we will need them together with
the logbuf_lock_*() stuff.

> [..]
> > PS: I still think if we could come with a better name than
> > printk_safe() but I cannot find one.
> 
> well, not that I'm the fan of printk_safe name, but can't think
> of anything better. we make printk calls safe (deadlock safe) in
> places where previously it was unsafe... quick-&-dirty name that
> is implementation-specific -- printk_percpu_enter/exit, or
> printk_pcpu_enter/exit... dunno.

OK, let's stay with printk_safe :-)

Best Reagards,
Petr