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

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Message-ID: <20150416180932.GW889@ZenIV.linux.org.uk>
Date:	Thu, 16 Apr 2015 19:09:32 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	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, Apr 16, 2015 at 02:16:31PM +0200, Mateusz Guzik wrote:
> @@ -165,8 +165,10 @@ static int expand_fdtable(struct files_struct *files, int nr)
>  	cur_fdt = files_fdtable(files);
>  	if (nr >= cur_fdt->max_fds) {
>  		/* Continue as planned */
> +		write_seqcount_begin(&files->fdt_seqcount);
>  		copy_fdtable(new_fdt, cur_fdt);
>  		rcu_assign_pointer(files->fdt, new_fdt);
> +		write_seqcount_end(&files->fdt_seqcount);
>  		if (cur_fdt != &files->fdtab)
>  			call_rcu(&cur_fdt->rcu, free_fdtable_rcu);

Interesting.  AFAICS, your test doesn't step anywhere near that path,
does it?  So basically you never hit the retries during that...
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