linux-kernel - Re: f_owner.lock and file->pos updates

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Message-ID: <f2b55d220703071812l52ef7241h9f2729a9b48ab60d@mail.gmail.com>
Date:	Wed, 7 Mar 2007 18:12:40 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: f_owner.lock and file->pos updates

I wrote:
> I didn't see any clean way to intersperse overwrites and appends to a
> record-structured file without using vfs_llseek, which steps on f_pos.

The context, of course, is an attempt to fix -ENOPATCH with regard to
the netlink-based AIO submission scheme I outlined a couple of days
ago.  :-)

Maybe f_pos should be advanced atomically by the number of bytes
expected to be read/written, before entering the vfs_(read|write)(|v)
call?  And then if the read/write doesn't complete normally, f_pos
should be decremented by the number of bytes we failed to read/write?
Or do we have to make absolutely, positively sure that sampling f_pos
from another thread never returns any value outside (before)..(before
+ bytes read/written)?  If so, the only way to cure the worst symptom
of the append race appears to be to hold a per-fd lock for the
duration of the sys_(read|write).

Cheers,
- Michael
-
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