Date: Thu, 16 Feb 2012 15:49:54 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Vasiliy Kulikov <segoon@openwall.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Andrey Vagin <avagin@openvz.org>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Glauber Costa <glommer@parallels.com>,
        Andi Kleen <andi@firstfloor.org>, Tejun Heo <tj@kernel.org>,
        Matt Helsley <matthltc@us.ibm.com>, Pekka Enberg <penberg@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>, Valdis.Kletnieks@vt.edu,
        Michal Marek <mmarek@suse.cz>,
        Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree
Message-ID: <20120216144954.GA11953@redhat.com>
References: <20120215161329.GM1894@moon> <20120215162222.GA18266@redhat.com> <20120215175319.GG4533@moon> <20120215184336.GA24182@redhat.com> <20120215195610.GJ4533@moon> <20120215195733.GA8021@albatros> <20120215200533.GQ1894@moon> <20120215202538.GK4533@moon> <20120215210934.GL4533@moon> <20120215215807.GM4533@moon>
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On 02/16, Cyrill Gorcunov wrote:
>
> +static int access_trylock(struct task_struct *task)
> +{
> +	if (!mutex_trylock(&task->signal->cred_guard_mutex))
> +		return -EBUSY;
> +
> +	if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
> +		mutex_unlock(&task->signal->cred_guard_mutex);
> +		return -EPERM;
> +	}
> +
> +	return 0;
> +}

OK, this looks correct, but I don't really understand _trylock.
This means the caller should always retry if -EBUSY, and
kcmp(pid, pid) can never succeed. Sure, kcmp() doesn't make
a lot of sense if pid1 == pid2, but this looks a bit strange.

You could simply do

	int mutex_double_lock_killable(struct mutex *m1, struct mutex *m2)
	{
		int err;

		if (m2 > m1)
			swap(m1, m2);

		err = mutex_lock_killable(m1);

		if (!err && likely(m1 != m2)) {
			err = mutex_lock_killable_nested(m2);
			if (err)
				mutex_unlock(m1);
		}

		return err;
	}

but I won't unsist.

Oleg.

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