Messages in this thread | | | Subject | Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers | From | Rasmus Villemoes <> | Date | Thu, 24 Sep 2020 11:49:06 +0200 |
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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
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