Re: [BUG] fs/dcache: might_sleep is called under a spinlock

From: Jia-Ju Bai
Date: Tue Oct 03 2017 - 04:47:00 EST

Thanks for your detailed explanation :)
I will improve my static analysis tool.

Jia-Ju Bai

On 2017/10/3 11:19, Al Viro wrote:
On Tue, Oct 03, 2017 at 10:38:25AM +0800, Jia-Ju Bai wrote:
According to fs/dcache.c, might_sleep is called under a spinlock,
and the function call path is:
d_prune_aliases (acquire the spinlock)

This bug is found by my static analysis tool and my code review.
A possible fix is to remove might_sleep in dput.
... or to fix your static analysis tool. First of all, that call
of dput() really *can* block and if we had inode->i_lock or dentry->d_lock
still held at that point we'd have a real bug. However, __dentry_kill()
there is called with dentry->d_inode == inode and inode->i_lock held,
so dentry->d_inode is stable until inode->i_lock is dropped. Said
__dentry_kill() contains
if (dentry->d_inode)
with inode->i_lock held until that point. dentry_unlink_inode() starts
struct inode *inode = dentry->d_inode;
bool hashed = !d_unhashed(dentry);

if (hashed)
if (hashed)
1) inode in there is guaranteed to be equal to the argument of
d_prune_aliases() and
2) both dentry->d_lock and inode->i_lock are dropped before
dentry_unlink_inode() returns. inode->i_lock is not regained in the
rest of __dentry_kill(); dentry->d_lock is regained and dropped before
__dentry_kill() returns. IOW, we are fine - dput() in d_prune_aliases()
is called without any spinlocks held.

That, BTW, is the reason for
goto restart;
in there, instead of just continuing the loop - if we get to that point,
the list of aliases might have changed.

Removing might_sleep() in dput() would've been wrong - it really might
sleep when called from that point. Here's how: we used to have two
links to the same file - foo/bar and baz/barf. baz/barf used to be
opened, then rm -rf baz happened and later we'd called d_prune_aliases()
on the inode of foo/bar. And as the loop had been executed on one CPU,
on another the opened file got closed, dropping the last reference to
dentry that used to be baz/barf. Note that its parent (the thing that
used to be dentry of baz) is unhashed and the only contributor to its
refcount is our dentry, so dput(parent) *does* drop the last remaining
reference, triggering the final iput() on inode of baz, along with
freeing on-disk inode, doing disk IO, etc.

Again, it's not that we can't block in that dput() - it's that __dentry_kill()
drops all spinlocks.