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kernfs: fix locking around kernfs_ops->release() callback

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The release callback may be called from two places - file release
operation and kernfs open file draining.  kernfs_open_file->mutex is
used to synchronize the two callsites.  This unfortunately leads to
possible circular locking because of->mutex is used to protect the
usual kernfs operations which may use locking constructs which are
held while removing and thus draining kernfs files.

@of->mutex is for synchronizing concurrent kernfs access operations
and all we need here is synchronization between the releaes and drain
paths.  As the drain path has to grab kernfs_open_file_mutex anyway,
let's use the mutex to synchronize the release operation instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Tony Lindgren <tony@atomide.com>
Fixes: 0e67db2f9f ("kernfs: add kernfs_ops->open/release() callbacks")
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Tejun Heo 2017-02-12 05:33:02 +09:00
parent 63f1ca5945
commit f83f3c5156
1 changed files with 14 additions and 5 deletions
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19
fs/kernfs/file.c
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@ -747,10 +747,15 @@ err_out:
static void kernfs_release_file(struct kernfs_node *kn,
struct kernfs_open_file *of)
{
if (!(kn->flags & KERNFS_HAS_RELEASE))
return;
/*
* @of is guaranteed to have no other file operations in flight and
* we just want to synchronize release and drain paths.
* @kernfs_open_file_mutex is enough. @of->mutex can't be used
* here because drain path may be called from places which can
* cause circular dependency.
*/
lockdep_assert_held(&kernfs_open_file_mutex);
mutex_lock(&of->mutex);
if (!of->released) {
/*
* A file is never detached without being released and we
@ -760,7 +765,6 @@ static void kernfs_release_file(struct kernfs_node *kn,
kn->attr.ops->release(of);
of->released = true;
}
mutex_unlock(&of->mutex);
}
static int kernfs_fop_release(struct inode *inode, struct file *filp)
@ -768,7 +772,12 @@ static int kernfs_fop_release(struct inode *inode, struct file *filp)
struct kernfs_node *kn = filp->f_path.dentry->d_fsdata;
struct kernfs_open_file *of = kernfs_of(filp);
kernfs_release_file(kn, of);
if (kn->flags & KERNFS_HAS_RELEASE) {
mutex_lock(&kernfs_open_file_mutex);
kernfs_release_file(kn, of);
mutex_unlock(&kernfs_open_file_mutex);
}
kernfs_put_open_node(kn, of);
seq_release(inode, filp);
kfree(of->prealloc_buf);