linux-kernel - Re: [PATCH 4/18] 2.6.17.9 perfmon2 patch for review: new system calls support

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Date:	Tue, 29 Aug 2006 15:15:18 -0700
From:	Stephane Eranian <eranian@....hp.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/18] 2.6.17.9 perfmon2 patch for review: new system calls support

On Tue, Aug 29, 2006 at 10:26:27AM -0700, Andrew Morton wrote:
> On Tue, 29 Aug 2006 09:59:57 -0700
> Stephane Eranian <eranian@....hp.com> wrote:
> 
> > > > +
> > > > +		PFM_DBG("going wait_inactive for [%d] state=%ld flags=0x%lx",
> > > > +			task->pid,
> > > > +			task->state,
> > > > +			local_flags);
> > > > +
> > > > +		spin_unlock_irqrestore(&ctx->lock, local_flags);
> > > > +
> > > > +		wait_task_inactive(task);
> > > > +
> > > > +		spin_lock_irqsave(&ctx->lock, new_flags);
> > > 
> > > This sort of thing..
> > 
> > We need to wait until the task is effectively off the CPU, i.e., with its
> > machine state (incl PMU) saved. When we come back we re-run the series of tests.
> > This applies only to per-thread, therefore it is not affected by smp_processor_id().
> > 
> 
> Generally, if a reviewer asks a question and the developer answers that
> question, this is a sign that a code comment is needed.  Because others are
> likely to ask themselves the same question ;)

Understood. I have added a comment about what is going there.

Now, I don't particularly like what we have to do here. You may have a better
solution to my problem:
	- how inside the kernel, can I make sure a thread is stopped and off any CPU?

	- if the thread is not stopped, how do I go about forcing it to stop and how
	  can I get notified when it is eventually off any CPU?

	- how do I make sure it cannot wake up until I am done with it?

> > > 
> > > When copying a struct from kernel to userspace we must beware of
> > > compiler-inserted padding.  Because that can cause the kernel to leak
> > > a few bytes of uninitialised kernel memory.
> > 
> > We are copying out exactly the same amount of data that was passed in.
> > 
> > Are you suggesting that copy_from/copy_to may copy more data?
> 
> No, that's OK.  I was pointing out the problem in situations like this:
> 
> struct foo {
> 	u32 a;
> 	u64 b;
> };
> 
> {
> 	struct foo f;
> 
> 	f.a = 0;
> 	f.b = 0;
> 	copy_to_user(p, &f, sizeof(f));
> }
> 
> which exposes kernel memory.  As you appear to be confident that the
> perfmon code can't do this, all is OK.

Ok, now I understand. I don't think I have that but I will write a test program
and run on all architectures to ensure this is indeed the case.

> 

-- 

-Stephane
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