[PATCH] 2.4 devfs deadlock on concurrent lookups on non-existent entry

Andrey Borzenkov (arvidjaar@mail.ru)
Wed, 7 May 2003 22:55:28 +0400

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This problem has first been reported for over two years ago. Usually it
happened during system boot using RH initscripts with two minilogds hanging
on access to /dev/log and blocking rc.sysinit; this condition was triggered
by unrelated bug in minilogd.

Pavel Roskin provided detailed debug info for this problem; details including
stack are available at
<https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=85621>. It turned out a
deadlock between devfs_lookup and devfs_d_revalidate_wait for cases when
->d_revalidate was called under parent->i_sem (mostly all places where
lookup_hash was used). The deadlock looked like:

path_lookup("dev/log", LOOKUP_PARENT, &nd);
-> yields dentry for </dev>

path_lookup("dev/log", LOOKUP_PARENT, &nd);
-> yields dentry for </dev>
down(</dev>->i_sem) holds i_sem

down(</dev>->i_sem) - sleeps

lookup_hash("log", </dev>) .
devfs_lookup(</dev>, "log") .
set "log"->d_op to &devfs_wait_dops; .
init "log"->wait_queue .
up(</dev>->i_sem) .
obtains i_sem
lookup_hash("log", </dev>);
cached_lookup(</dev>, "log", 0)
devfs_d_revalidate_wait("log", 0)
wait on "log"->wait_queue
... waits to be waked up by devfs_lookup


the patch fixes it by moving i_sem re-acquire after
wake_up(&lookup_info.wait_queue). It does not look like it adds any
additional races. Please check.

Pavel said it applies unchanged to 2.5 tree. Which may indicate it has the
same race condition. I do not have 2.5 available.

please consider for 2.4.21


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--- linux-2.4.21-0.13mdk/fs/devfs/base.c.minilogd 2002-11-29 02:53:15.000000000 +0300
+++ linux-2.4.21-0.13mdk/fs/devfs/base.c 2003-05-06 00:41:35.000000000 +0400
@@ -3038,7 +3038,6 @@ static struct dentry *devfs_lookup (stru
revalidation */
up (&dir->i_sem);
wait_for_devfsd_finished (fs_info); /* If I'm not devfsd, must wait */
- down (&dir->i_sem); /* Grab it again because them's the rules */
de = lookup_info.de;
/* If someone else has been so kind as to make the inode, we go home
early */
@@ -3068,6 +3067,7 @@ out:
write_lock (&parent->u.dir.lock);
wake_up (&lookup_info.wait_queue);
write_unlock (&parent->u.dir.lock);
+ down (&dir->i_sem); /* Grab it again because them's the rules */
devfs_put (de);
return retval;
} /* End Function devfs_lookup */


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