linux-kernel - Re: [PATCH v4 04/29] nios2: Exception handling

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Date:	Wed, 8 Oct 2014 23:27:22 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ley Foon Tan <lftan@...era.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	lftan.linux@...il.com, cltang@...esourcery.com
Subject: Re: [PATCH v4 04/29] nios2: Exception handling


> +Lsignal_return:
> +	ANDI32	r1, r10, _TIF_SIGPENDING | _TIF_NOTIFY_RESUME
> +	beq	r1, r0, restore_all
> +	mov	r4, sp			/* pt_regs */
> +	SAVE_SWITCH_STACK
> +	call	do_notify_resume
> +	beq	r2, r0, no_work_pending
> +	RESTORE_SWITCH_STACK
> +	/* prepare restart syscall here without leaving kernel */
> +	ldw	r2, PT_R2(sp)	/* reload syscall number in r2 */
> +	ldw 	r4, PT_R4(sp)	/* reload syscall arguments r4-r9 */
> +	ldw 	r5, PT_R5(sp)
> +	ldw 	r6, PT_R6(sp)
> +	ldw 	r7, PT_R7(sp)
> +	ldw 	r8, PT_R8(sp)
> +	ldw 	r9, PT_R9(sp)
> +	br	local_restart	/* restart syscall */
> +
> +no_work_pending:
> +	RESTORE_SWITCH_STACK
> +	br	restore_all

Umm...  Branch to no_work_pending will be usuall taken.  How well does
branch predictor on that processor cope with such things?

Another thing is the point Richard made - multiple pending signals need to be
handled.  IOW, that should be a branch to ret_from_exception, not restore_all.
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