linux-kernel - Re: [PATCH] TPM: Let the tpm char device be openable multiple times

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Message-ID: <20091103171538.GP1966@obsidianresearch.com>
Date:	Tue, 3 Nov 2009 10:15:38 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	tpmdd-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	srajiv@...ux.vnet.ibm.com
Subject: Re: [PATCH] TPM: Let the tpm char device be openable multiple times

On Tue, Nov 03, 2009 at 10:43:52AM +0000, Alan Cox wrote:
> > - Move the user space data buffer and related goop into a
> >   struct tpm_file, create one struct tpm_file per open file.
> 
> Is that really sufficient ?

Yes, I think so. Basically, all I did was take the per-device locking
that was already there and make it per-file. The locking hasn't
changed. If there was a multi-threading bug in the old code, it is
still in this version - but I looked for such a thing and didn't
see anything.

> You can open a file once, dup it (or inherit it and get multiple callers
> from the same file handle. Ditto consider threated apps

Yes, I did, I think it is OK. All the operations involving the file's
buffer are atomic or protected by a per-file mutex. Do you see
something specific?

It would be dangerous to do that anyhow, the way the TPM interface
works requires matched write/read pairs. The driver has a hacky way to
halt a write until the matching write happens, but it is based on
timeouts and probably isn't 100% reliable.

Thanks,
Jason
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