linux-kernel - Re: Conditional Unlink

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Message-ID: <20120924150239.1d87b868@notabene.brown>
Date:	Mon, 24 Sep 2012 15:02:39 +1000
From:	NeilBrown <neilb@...e.de>
To:	"Fox, Kevin M" <kevin.fox@...l.gov>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Conditional Unlink

On Fri, 21 Sep 2012 10:32:36 -0700 "Fox, Kevin M" <kevin.fox@...l.gov> wrote:

> Quick question:
> Is there a syscall or some algorithm that would allow an unlink that would only happen if no writes to the file have occurred between a stat and an unlink, race free?
> 
> Background:
> I'm writing a program that sync's files up to a server. I would like to unlink the files that have successfully been uploaded. There is a race though. If the file changed after uploading but before the unlink (I do not control who can write to the files), the next upload should take care of things and the unlink should fail. Otherwise, data could be lost!

Probably your best bet is to rename the file in there.
 mv file file.tmp
 sleep 5
 cp file.tmp $server/file
 rm file.tmp

The "sleep 5" might not work for you, one would need to know more about total
context.

Is any locking done when writing to the file, or will there only ever be one
writer?

NeilBrown

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