From owner-freebsd-current@FreeBSD.ORG  Wed Aug  3 05:16:39 2011
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Date: Wed, 03 Aug 2011 14:16:36 +0900 (JST)
Message-Id: <20110803.141636.145758949453810779.okuno.kohji@jp.panasonic.com>
To: freebsd-current@freebsd.org
From: Kohji Okuno <okuno.kohji@jp.panasonic.com>
Organization: Panasonic Corporation
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Cc: okuno.kohji@jp.panasonic.com
Subject: Bug: devfs is sure to have the bug.
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Hello,

I think that devfs is sure to have the bug.
I found that I couldn't open "/dev/XXX" though the kernel detected XXX
device.


"dm->dm_generation" is updated with "devfs_generation" in
devfs_populate(), and the context holds only "dm->dm_lock" in
devfs_populate().

On the other hand, "devfs_generation" is incremented in devfs_create()
and devfs_destroy() the context holds only "devmtx" in devfs_create()
and devfs_destroy().

If a context executes devfs_create() when other context is executing
(***), then "dm->dm_generation" is updated incorrect value.
As a result, we can not open the last detected device (we receive ENOENT).

I think that we should change the lock method.
May I have advice?

void
devfs_populate(struct devfs_mount *dm)
{

        sx_assert(&dm->dm_lock, SX_XLOCKED);
        if (dm->dm_generation == devfs_generation)
                return;
        while (devfs_populate_loop(dm, 0))
                continue;
(***)
        dm->dm_generation = devfs_generation;
}

void
devfs_create(struct cdev *dev)
{
        struct cdev_priv *cdp;

        mtx_assert(&devmtx, MA_OWNED);
        cdp = cdev2priv(dev);
        cdp->cdp_flags |= CDP_ACTIVE;
        cdp->cdp_inode = alloc_unrl(devfs_inos);
        dev_refl(dev);
        TAILQ_INSERT_TAIL(&cdevp_list, cdp, cdp_list);
        devfs_generation++;
}

void
devfs_destroy(struct cdev *dev)
{
        struct cdev_priv *cdp;

        mtx_assert(&devmtx, MA_OWNED);
        cdp = cdev2priv(dev);
        cdp->cdp_flags &= ~CDP_ACTIVE;
        devfs_generation++;
}

Thanks.
--
 Kohji Okuno (okuno.kohji@jp.panasonic.com)