linux-kernel - Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

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Date:   Thu, 24 Sep 2020 11:49:06 +0200
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

On 24/09/2020 10.53, Sergey Senozhatsky wrote:
> On (20/09/24 10:45), Petr Mladek wrote:
>> On Thu 2020-09-24 14:40:58, Sergey Senozhatsky wrote:
>>> On (20/09/23 17:11), Petr Mladek wrote:
>>>>
>>>> AFAIK, there is one catch. We need to use va_copy() around
>>>> the 1st call because va_format can be proceed only once.
>>>>
>>>
>>> Current printk() should be good enough for reporting, say, "Kernel
>>> stack overflow" errors. Is extra pressure that va_copy() adds something
>>> that we need to consider?
>>
>> The thing is that vsprintf() traverses the arguments using va_arg().
>> It modifies internal values so that the next va_arg() will read
>> the next value.
> 
> Yes, I understand the purpose of va_copy(). I'm asking if we are
> always on the safe side doing va_copy for every printk (+ potential
> recursive va_copy-s).

va_copy doesn't really add any extra stack use worth talking about. When
ABI says all arguments are passed on stack, va_list is just a pointer
into the arguments, and va_copy merely copies that pointer. When some
arguments are passed in register, the function prologue sets up a
"register save area" where those registers are stashed, and va_list then
contains two pointers: one to the reg save area, one to the place in the
stack where the rest of the arguments are, plus a bit of control
information on how many of the register arguments have been consumed so
far (and that control info is the only reason one must "reset" a
va_list, or rather use a copy that was made before consumption started).
In either case, an extra va_list doesn't cost more than 24 bytes or so
of stack - even if printk() was called with 100 arguments, those
90-100ish arguments are only stored once on the stack.

Rasmus