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Date: Fri, 27 Jul 2012 20:04:23 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Ingo Molnar <mingo@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH] uprobes: don't enable/disable signle step if the user
	did it
Message-ID: <20120727180423.GA21022@redhat.com>
References: <1343316043-13475-1-git-send-email-bigeasy@linutronix.de> <20120726173126.GA5787@redhat.com> <5012D25B.3040302@linutronix.de>
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On 07/27, Sebastian Andrzej Siewior wrote:
>
> On 07/26/2012 07:31 PM, Oleg Nesterov wrote:
>> Well. I agree, this needs changes. To begin with, uprobe should avoid
>> user_enable_single_step() which does access_process_vm(). And I suspect
>> uprobes have the problems with TIF_FORCED_TF logic.
>
> Why? Shouldn't wee keep the trap flag if the instruction on which we
> placed the uprobe activates it?

Yes. But user_enable_single_step() is not the right interface.

>> But I am not sure about this patch...
>>
>> On 07/26, Sebastian Andrzej Siewior wrote:
>>>
>>> @@ -1528,7 +1528,10 @@ static void handle_swbp(struct pt_regs *regs)
>>>
>>>   	utask->state = UTASK_SSTEP;
>>>   	if (!pre_ssout(uprobe, regs, bp_vaddr)) {
>>> -		user_enable_single_step(current);
>>> +		if (test_tsk_thread_flag(current, TIF_SINGLESTEP))
>>> +			uprobe->flags |= UPROBE_USER_SSTEP;
>>> +		else
>>> +			user_enable_single_step(current);
>>
>> This is x86 specific, TIF_SINGLESTEP is not defined on every arch.
>
> It is not defined on every arch but I wouldn't say it is 86 specific.
> From the architectures which have user_enable_single_step() defined I
> see

But we do not need TIF_SINGLESTEP. At all. Again, this is ptrace thing
connected to user_enable_single_step().

Sebastian, I am sorry for being terse, I'll write another email later.

Oleg.