linux-kernel - Re: [PATCH 0/2] rcu_dereference_check

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Message-ID: <20140108151918.GA5156@redhat.com>
Date:	Wed, 8 Jan 2014 16:19:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] rcu_dereference_check_fdtable fix/cleanups

On 01/08, Paul E. McKenney wrote:
>
> I am not all that excited about invoking rcu_lock_acquire() outside
> of RCU...

Yes, me too. That is why I thought about the helper with a good name,
see below.

> Another approach would be to add an argument to files_fdtable()
> that is zero normally and one for "we know we don't need RCU
> protection."  Then rcu_dereference_check() could be something
> like the following:
>
> #define files_fdtable(files, c) \
> 		(rcu_dereference_check_fdtable((files), (files)->fdt) || c)
>
> Would that work?

Yes, I considered this optiion, but this needs much more uglifications^W
changes.

Either we need to change all users of files_fdtable(), or we need something
like

	#define __rcu_dereference_check_fdtable(files, unshared, fdtfd) \
		rcu_dereference_check((fdtfd), unshared || lockdep_is_held(&(files)->file_lock))

	#define rcu_dereference_check_fdtable(files, fdtfd)
		__rcu_dereference_check_fdtable(files, false, fdtfd)

	#define __files_fdtable(files)
		__rcu_dereference_check_fdtable((files), true, (files)->fdt)

	#define files_fdtable(files)
		__rcu_dereference_check_fdtable((files), false, (files)->fdt)

Plus we need

	static inline struct file *__fcheck_files(struct files_struct *files,
						bool unshared, unsigned int fd)
	{
		struct file *file = NULL;
		struct fdtable *fdt = __rcu_dereference_check_fdtable(files, unshared, files->fdt);

		if (fd < fdt->max_fds)
			file = __rcu_dereference_check_fdtable(files, unshared, fdt->fd[fd]);
		return file;
	}

doesn't look very nice...

As for 2/2, probably close_files() can simply do

	/*
	 * It is safe to dereference the fd table without RCU or
	 * ->file_lock because this is the last reference to the
	 * files structure.
	 */
	fdt = rcu_dereference_raw(files->fdt);

Or we can add

	#define __files_fdtable(files)	\
		 rcu_dereference_raw((files)->fdt)

but it is not clear to me what 1/1 should do. Perhaps

	static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd)
	{
		struct fdtable *fdt = rcu_dereference_raw(files->fdt);
		struct file *file = NULL;

		if (fd < fdt->max_fds)
			file = rcu_dereference_raw(fdt->fd[fd]);

		return file;
	}

	static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd)
	{
		rcu_lockdep_assert(rcu_read_lock_held() ||
				   lockdep_is_held(files->file_lock),
				   "message");
		return __fcheck_files(files, fd);
	}

?

Oleg.

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