linux-kernel - Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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Date:	Thu, 16 Feb 2012 15:49:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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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