From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pekka Enberg <penberg@cs.helsinki.fi>
Subject: Re: [PATCH 12/17] [LogFS] readwrite.c
Date: Mon, 23 Nov 2009 14:33:15 +0200
Message-ID: <84144f020911230433q5fd90321m9b463cb425fafe7d@mail.gmail.com>
References: <20091120181113.GA2159@logfs.org>
	 <E1NBZJ5-0003hL-Qc@longford.logfs.org>
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Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mtd@lists.infradead.org, Arnd Bergmann <arnd@arndb.de>,
	Hugh Dickins <hugh.dickins@tiscali.co.uk>,
	Andrew Morton <akpm@linux-foundation.org>
To: Joern Engel <joern@logfs.org>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
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In-Reply-To: <E1NBZJ5-0003hL-Qc@longford.logfs.org>
Sender: linux-fsdevel-owner@vger.kernel.org
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Hi Joern,

(Dunno who to CC, really, so lets see if I can trick Andrew or Hugh
into looking at the issue.)

On Fri, Nov 20, 2009 at 9:38 PM, Joern Engel <joern@logfs.org> wrote:
> +static void logfs_lock_write_page(struct page *page)
> +{
> + =A0 =A0 =A0 int loop =3D 0;
> +
> + =A0 =A0 =A0 while (unlikely(!trylock_page(page))) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (loop++ > 0x1000) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 /* Has been observed on=
ce so far... */
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 printk(KERN_ERR "stack =
at %p\n", &loop);
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 BUG();
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (PagePreLocked(page)) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 /* Holder of page lock =
is waiting for us, it
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* is safe to use thi=
s page. */
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 break;
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 /* Some other process has this page loc=
ked and has
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* nothing to do with us. =A0Wait for=
 it to finish.
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*/
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 schedule();
> + =A0 =A0 =A0 }
> + =A0 =A0 =A0 BUG_ON(!PageLocked(page));
> +}

What's the purpose of PagePreLocked()? The above function looks pretty
fragile for a filesystem to me.

                        Pekka
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