linux-kernel - Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap

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Date:	Wed, 20 May 2009 21:03:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap_copyout())

On 05/11, Ingo Molnar wrote:
>
> See the (new) put_user_try / put_user_ex() / put_user_catch()
> abstraction in arch/x86/include/asm/uaccess.h, and how all the x86
> signal code makes use of that to optimize such patterns of per field
> user copies.

Just curious, can't we simplify put_user_{try,ex,catch} ?

Pseudo-code:

	#define put_user_try				\
		do {					\
			__label__  __efault_label;	\


	#define put_user_catch(err)			\
			err = 0;			\
			if (0) {			\
			__efault_label:			\
				err = -EFAULT;		\
			}				\
		while (0)
		

	#define __put_user_asm_ex(...)				\
		asm volatile(					\
			"1:	mov ..."			\
			_ASM_EXTABLE(1b, &__efault_label)	\
		     	: : ...)


Now, we don't need thread_info->uaccess_err, and we don't need the
special "if (fixup->fixup < 16)" hack in fixup_exception(). Once
any put_user_ex() fails, we jump to the __efault_label and set
err = -EFAULT.

This also means that we skip other put_user_ex's after the faulted
one. Not very important, this is unlikely case, but imho nice anyway.

Can this work? (warning: my asm skills is almost zero ;)

Oleg.

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