linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Date:	Fri, 17 Apr 2015 14:46:56 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

On Thu, 2015-04-16 at 14:16 +0200, Mateusz Guzik wrote:
> Hi,
> 
> Currently obtaining a new file descriptor results in locking fdtable
> twice - once in order to reserve a slot and second time to fill it

...


>  void __fd_install(struct files_struct *files, unsigned int fd,
>  		struct file *file)
>  {
> +	unsigned long seq;

	unsigned int seq;

>  	struct fdtable *fdt;
> -	spin_lock(&files->file_lock);
> -	fdt = files_fdtable(files);
> -	BUG_ON(fdt->fd[fd] != NULL);
> -	rcu_assign_pointer(fdt->fd[fd], file);
> -	spin_unlock(&files->file_lock);
> +
> +	rcu_read_lock();
> +	do {
> +		seq = read_seqcount_begin(&files->fdt_seqcount);
> +		fdt = files_fdtable_seq(files);
> +		/*
> +		 * Entry in the table can already be equal to file if we
> +		 * had to restart and copy_fdtable picked up our update.
> +		 */
> +		BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file));
> +		rcu_assign_pointer(fdt->fd[fd], file);
> +		smp_mb();
> +	} while (__read_seqcount_retry(&files->fdt_seqcount, seq));
> +	rcu_read_unlock();
>  }
>  

So one problem here is :

As soon as  rcu_assign_pointer(fdt->fd[fd], file) is done, and other cpu
does one expand_fdtable() and releases files->file_lock, another cpu can
close(fd).

Then another cpu can reuse the [fd] now empty slot and install a new
file in it.

Then this cpu will crash here :

BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file));



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