linux-kernel - Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file

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Date:	Tue, 02 Jun 2009 15:56:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Nick Piggin <npiggin@...e.de>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Tejun Heo <tj@...nel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file

Nick Piggin <npiggin@...e.de> writes:

>> In addition for a complete solution we need:
>> - A reliable way the file structures that we need to revoke.
>> - To wait for but not tamper with ongoing file creation and cleanup.
>> - A guarantee that all with user space controlled duration are removed.
>> 
>> The file_hotplug_lock has a very unique implementation necessitated by
>> the need to have no performance impact on existing code.  Classic locking
>
> Well, it isn't no performance impact. Function calls, branches, icache
> and dcache...

Practically none.

Everything I could measure was in the noise.  It is cheaper than any serializing
locking primitive.  I ran both lmbench and did some microbenchmark testing.
So I know on the fast path the overhead is minimal.  Certainly less than  what
we are doing in sysfs and proc today.

Eric

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