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Date: Fri, 18 Jul 2008 14:52:29 -0400
From: Jeff Moyer <jmoyer@...hat.com>
To: Ian Kent <raven@...maw.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
autofs mailing list <autofs@...ux.kernel.org>,
Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Al Viro <viro@...IV.linux.org.uk>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/7] autofs4 - fix indirect mount pending expire race
Ian Kent <raven@...maw.net> writes:
> The selection of a dentry for expiration and the setting of the
> AUTOFS_INF_EXPIRING flag isn't done atomically which can lead to
> lookups walking into an expiring mount.
>
> What happens is that an expire is initiated by the daemon and
> a dentry is selected for expire but, since there is no lock
> held between the selection and setting of the expiring flag,
> a process may find the flag clear and continue walking into
> the mount tree at the same time the daemon attempts the expire
> it.
>
> Signed-off-by: Ian Kent <raven@...maw.net>
>
> ---
[...]
> static inline void autofs4_copy_atime(struct file *src, struct file *dst)
> diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
> index 894fee5..19f5bea 100644
> --- a/fs/autofs4/expire.c
> +++ b/fs/autofs4/expire.c
> @@ -292,6 +292,8 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> struct list_head *next;
> int do_now = how & AUTOFS_EXP_IMMEDIATE;
> int exp_leaves = how & AUTOFS_EXP_LEAVES;
> + struct autofs_info *ino;
> + unsigned int ino_count;
>
> if (!root)
> return NULL;
> @@ -316,6 +318,9 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> dentry = dget(dentry);
> spin_unlock(&dcache_lock);
>
> + spin_lock(&sbi->fs_lock);
> + ino = autofs4_dentry_ino(dentry);
> +
> /*
> * Case 1: (i) indirect mount or top level pseudo direct mount
> * (autofs-4.1).
> @@ -326,6 +331,11 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> DPRINTK("checking mountpoint %p %.*s",
> dentry, (int)dentry->d_name.len, dentry->d_name.name);
>
> + /* Path walk currently on this dentry? */
> + ino_count = atomic_read(&ino->count) + 2;
> + if (atomic_read(&dentry->d_count) > ino_count)
> + goto next;
> +
It would be nice to document the +2. The +1s (below) may be evident
given that we did a dget above, but still might merit mention.
> /* Can we umount this guy */
> if (autofs4_mount_busy(mnt, dentry))
> goto next;
> @@ -343,23 +353,25 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
>
> /* Case 2: tree mount, expire iff entire tree is not busy */
> if (!exp_leaves) {
> - /* Lock the tree as we must expire as a whole */
> - spin_lock(&sbi->fs_lock);
> - if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
> - struct autofs_info *inf = autofs4_dentry_ino(dentry);
> + /* Path walk currently on this dentry? */
> + ino_count = atomic_read(&ino->count) + 1;
> + if (atomic_read(&dentry->d_count) > ino_count)
> + goto next;
>
> - /* Set this flag early to catch sys_chdir and the like */
> - inf->flags |= AUTOFS_INF_EXPIRING;
> - spin_unlock(&sbi->fs_lock);
> + if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
> expired = dentry;
> goto found;
> }
> - spin_unlock(&sbi->fs_lock);
> /*
> * Case 3: pseudo direct mount, expire individual leaves
> * (autofs-4.1).
> */
> } else {
> + /* Path walk currently on this dentry? */
> + ino_count = atomic_read(&ino->count) + 1;
> + if (atomic_read(&dentry->d_count) > ino_count)
> + goto next;
> +
> expired = autofs4_check_leaves(mnt, dentry, timeout, do_now);
> if (expired) {
> dput(dentry);
> @@ -367,6 +379,7 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> }
> }
> next:
> + spin_unlock(&sbi->fs_lock);
> dput(dentry);
> spin_lock(&dcache_lock);
> next = next->next;
> @@ -377,6 +390,9 @@ next:
> found:
> DPRINTK("returning %p %.*s",
> expired, (int)expired->d_name.len, expired->d_name.name);
> + ino = autofs4_dentry_ino(expired);
If we get here, ino is already set to the autofs4_dentry_ino(expired),
so this statement is redundant.
[...]
> diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> index 2944b28..2ed2a51 100644
> --- a/fs/autofs4/root.c
> +++ b/fs/autofs4/root.c
> @@ -133,7 +133,10 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
> /* Block on any pending expiry here; invalidate the dentry
> when expiration is done to trigger mount request with a new
> dentry */
> - if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
> + spin_lock(&sbi->fs_lock);
> + if (ino->flags & AUTOFS_INF_EXPIRING) {
> + spin_unlock(&sbi->fs_lock);
> +
> DPRINTK("waiting for expire %p name=%.*s",
> dentry, dentry->d_name.len, dentry->d_name.name);
This is okay, since we wait on the AUTOFS_INF_EXPIRING flag in
validate_request. That check is done outside the lock, but I doubt
there are issues with not seeing an update since you perform a schedule
there.
> @@ -149,8 +152,11 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
> status = d_invalidate(dentry);
> if (status != -EBUSY)
> return -EAGAIN;
> - }
>
> + goto cont;
> + }
> + spin_unlock(&sbi->fs_lock);
> +cont:
> DPRINTK("dentry=%p %.*s ino=%p",
> dentry, dentry->d_name.len, dentry->d_name.name, dentry->d_inode);
>
> @@ -229,15 +235,21 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
> goto done;
>
> /* If an expire request is pending wait for it. */
> - if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
> + spin_lock(&sbi->fs_lock);
> + if (ino->flags & AUTOFS_INF_EXPIRING) {
> + spin_unlock(&sbi->fs_lock);
> +
> DPRINTK("waiting for active request %p name=%.*s",
> dentry, dentry->d_name.len, dentry->d_name.name);
>
> status = autofs4_wait(sbi, dentry, NFY_NONE);
>
> DPRINTK("request done status=%d", status);
> - }
>
> + goto cont;
> + }
> + spin_unlock(&sbi->fs_lock);
> +cont:
could've done an:
} else
spin_unlock(&sbi->fs_lock);
But, whatever...
Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
--
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