linux-kernel - Re: [patch 20/24] perfmon: system calls interface

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Date:	Thu, 27 Nov 2008 15:42:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	eranian@...il.com
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	x86@...nel.org, andi@...stfloor.org, sfr@...b.auug.org.au,
	Roland McGrath <roland@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch 20/24] perfmon: system calls interface

* stephane eranian <eranian@...glemail.com> wrote:

> Ingo,
> 
> On Wed, Nov 26, 2008 at 3:00 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > Thirdly, the check for ->exit_state in pfm_task_incompatible() is 
> > not needed: we've just passed ptrace_check_attach() so we know we 
> > just transitioned the task to task->state == TASK_TRACED.
> >
> > If you _ever_ see a task exit TASK_TRACED and go zombie or dead 
> > from there without this code allowing it that means the whole 
> > state machine with ptrace is borked up by perfmon. For example i 
> > dont see where the perfmon-control task parents itself as the 
> > exclusive debugger (parent) of the debuggee-task.
> >
> 
> Perfmon requires ptrace ONLY to stop the thread you want to operate 
> on. For instance, to read the counters in a thread via pfm_read(), 
> you need to have that thread stopped, so perfmon can extract the 
> machine state safely. But when the monitored thread runs, it does 
> not have to remain under the control of ptrace. All that is needed 
> is that the thread is stopped while we are in the perfmon syscall. I 
> think ptrace allows this today. We will be able to drop ptrace() 
> once we switch to utrace in which case, the kernel will be able to 
> easily stop the thread when entering the perfmon syscalls. I guess I 
> don't quite understand the meaning of your last sentence.

The meaning of my last sentence is the jist of my argument: you cannot 
do it like this! You are using a bit of the ptrace infrastructure but 
unsafely, as pointed out here.

and the thing is, i fail to understand the whole justification of the 
new sys_pfm_attach()/PFM_NO_TARGET system calls.

Firstly, there's a taste issue: why didnt you add sys_pfm_detach 
instead of adding a butt-ugly PFM_NO_TARGET special case into 
sys_pfm_attach() that maps to pfm_detach??

But more importantly, and very fundamentally: why did you implement it 
as a special system call? Why didnt you extend ptrace to read/write 
the PMU context? It is _trivial_ and needs no new syscalls at all: 
just a new ptrace parameter to arch_ptrace(). And ptrace will drive 
the TASK_TRACED state machine safely - it already stops/starts tasks 
to read/write hardware context safely.

And as a bonus, if this is implemented via a ptrace extension it will 
be trivial to add support for these new context types to all sorts of 
user-space debuggers as well. With new syscalls it will take ages for 
this to trickle through to all parties involved.

	Ingo
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