linux-kernel - Re: [RFC,PATCH 14/14] utrace core

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Message-ID: <20091208163131.GA14815@redhat.com>
Date:	Tue, 8 Dec 2009 17:31:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	Ananth Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, utrace-devel@...hat.com
Subject: Re: [RFC,PATCH 14/14] utrace core

On 12/08, Peter Zijlstra wrote:
>
> On Tue, 2009-12-08 at 16:04 +0100, Oleg Nesterov wrote:
> >
> > Well, this is subjective, but I don't agree that
> >
> >         get_task_struct(task);
> >         task->utrace_flags = flags;
> >         spin_unlock(&utrace->lock);
> >         put_task_struct(task);
> >
> > looks better.
>
> No, what I mean by assymetric locking is that utrace_reset() and
> utrace_reap() drop the utrace->lock where their caller acquired it,
> resulting in non-obvious like:
>
> utrace_control()
> {
>
>   ...
>   spin_lock(&utrace->lock);
>
>   ...
>
>   if (reset)
>     utrace_reset(utrace);
>   else
>     spin_unlock(&utrace->lock);
> }

Agreed, the code like this never looks good.

> If you take a task ref you can write the much saner:
>
> utrace_control()
> {
>   ...
>   spin_lock(&utrace->lock);
>   ...
>   if (reset)
>     utrace_reset(utrace);
>
>   spin_unlock(&utrace->lock);
> }

No, get_task_struct() in utrace_reset() can't help, we should move
it into utrace_control() then. And in this case it becomes even more
subtle: it is needed because ->utrace_flags may be cleared inside
utrace_reset() and after that utrace_control()->spin_unlock() becomes
unsafe.

Also. utrace_reset() drops utrace->lock to call put_detached_list()
lockless. If we want to avoid the assymetric locking, every caller
should pass "struct list_head *detached" to utrace_reset(), drop
utrace->lock, and call put_detached_list().

Oleg.

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