linux-kernel - Re: [RFC PATCH v1 11/31] ARC: Low level IRQ/Trap/Exception(non-MMU) Handling

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Date:	Thu, 27 Dec 2012 18:59:47 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 11/31] ARC: Low level IRQ/Trap/Exception(non-MMU)
 Handling

On Thursday 27 December 2012 02:30 PM, Vineet Gupta wrote:
> On Friday 16 November 2012 10:28 AM, Al Viro wrote:
>>> +	; --------- check for signals/restore-sigmask ------------
>>> +	bbit0  r9, TIF_SIGPENDING, chk_next_work
>>> +
>>> +	; save CALLEE Regs.
>>> +	; (i)  If this signal causes coredump - full regfile needed
>>> +	; (ii) If signal is SIGTRAP/SIGSTOP, task is being traced thus
>>> +	;      tracer might call PEEKUSR for a CALLEE reg
>>> +	;
>>> +	; NOTE: SP will grow up by size of CALLEE Reg-File
>>> +	SAVE_CALLEE_SAVED_USER		; clobbers r12
>>> +
>>> +	; save location of saved Callee Regs @ thread_struct->callee
>>> +	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r10
>>> +	st  sp, [r10, THREAD_CALLEE_REG]
>>> +
>>> +	bl  @do_signal
>>> +
>>> +	; unwind SP for cheap discard of Callee saved Regs
>>> +	DISCARD_CALLEE_SAVED_USER
>> Uh-oh...  And what if tracer wanted to modify callee-saved regs?
> 
> So the solution would be to either unconditionally restore all the 13 callee regs
> - or add additional state (struct thread) where ptrace makes a note that it wrote
> to a callee reg which is used here to conditional-ize the restore. Former is
> simpler to do - although it might ill-affect micro-benchmarks such as LMBench
> lat_sig. Anyhow correctness comes before optimization.

Havign coded above, I just couldn't accept the anti-optimization here. Is it
absolute sin to take a look at current->ptrace != 0 in deciding whether to
save/restore the regs. However that won't work for coredump case.
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