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NFS: add barriers when testing for NFS_FSDATA_BLOCKED

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dentry->d_fsdata is set to NFS_FSDATA_BLOCKED while unlinking or
renaming-over a file to ensure that no open succeeds while the NFS
operation progressed on the server.

Setting dentry->d_fsdata to NFS_FSDATA_BLOCKED is done under ->d_lock
after checking the refcount is not elevated.  Any attempt to open the
file (through that name) will go through lookp_open() which will take
->d_lock while incrementing the refcount, we can be sure that once the
new value is set, __nfs_lookup_revalidate() *will* see the new value and
will block.

We don't have any locking guarantee that when we set ->d_fsdata to NULL,
the wait_var_event() in __nfs_lookup_revalidate() will notice.
wait/wake primitives do NOT provide barriers to guarantee order.  We
must use smp_load_acquire() in wait_var_event() to ensure we look at an
up-to-date value, and must use smp_store_release() before wake_up_var().

This patch adds those barrier functions and factors out
block_revalidate() and unblock_revalidate() far clarity.

There is also a hypothetical bug in that if memory allocation fails
(which never happens in practice) we might leave ->d_fsdata locked.
This patch adds the missing call to unblock_revalidate().

Reported-and-tested-by: Richard Kojedzinszky <richard+debian+bugreport@kojedz.in>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1071501
Fixes: 3c59366c20 ("NFS: don't unhash dentry during unlink/rename")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
This commit is contained in:
NeilBrown 2024-05-28 13:27:17 +10:00 committed by Trond Myklebust
parent 33c94d7e3c
commit 99bc9f2eb3
1 changed files with 32 additions and 15 deletions
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47
fs/nfs/dir.c
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@ -1803,9 +1803,10 @@ __nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags,
if (parent != READ_ONCE(dentry->d_parent))
return -ECHILD;
} else {
/* Wait for unlink to complete */
/* Wait for unlink to complete - see unblock_revalidate() */
wait_var_event(&dentry->d_fsdata,
dentry->d_fsdata != NFS_FSDATA_BLOCKED);
smp_load_acquire(&dentry->d_fsdata)
!= NFS_FSDATA_BLOCKED);
parent = dget_parent(dentry);
ret = reval(d_inode(parent), dentry, flags);
dput(parent);
@ -1818,6 +1819,29 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
return __nfs_lookup_revalidate(dentry, flags, nfs_do_lookup_revalidate);
}
static void block_revalidate(struct dentry *dentry)
{
/* old devname - just in case */
kfree(dentry->d_fsdata);
/* Any new reference that could lead to an open
* will take ->d_lock in lookup_open() -> d_lookup().
* Holding this lock ensures we cannot race with
* __nfs_lookup_revalidate() and removes and need
* for further barriers.
*/
lockdep_assert_held(&dentry->d_lock);
dentry->d_fsdata = NFS_FSDATA_BLOCKED;
}
static void unblock_revalidate(struct dentry *dentry)
{
/* store_release ensures wait_var_event() sees the update */
smp_store_release(&dentry->d_fsdata, NULL);
wake_up_var(&dentry->d_fsdata);
}
/*
* A weaker form of d_revalidate for revalidating just the d_inode(dentry)
* when we don't really care about the dentry name. This is called when a
@ -2551,15 +2575,12 @@ int nfs_unlink(struct inode *dir, struct dentry *dentry)
spin_unlock(&dentry->d_lock);
goto out;
}
/* old devname */
kfree(dentry->d_fsdata);
dentry->d_fsdata = NFS_FSDATA_BLOCKED;
block_revalidate(dentry);
spin_unlock(&dentry->d_lock);
error = nfs_safe_remove(dentry);
nfs_dentry_remove_handle_error(dir, dentry, error);
dentry->d_fsdata = NULL;
wake_up_var(&dentry->d_fsdata);
unblock_revalidate(dentry);
out:
trace_nfs_unlink_exit(dir, dentry, error);
return error;
@ -2666,8 +2687,7 @@ nfs_unblock_rename(struct rpc_task *task, struct nfs_renamedata *data)
{
struct dentry *new_dentry = data->new_dentry;
new_dentry->d_fsdata = NULL;
wake_up_var(&new_dentry->d_fsdata);
unblock_revalidate(new_dentry);
}
/*
@ -2729,11 +2749,6 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
if (WARN_ON(new_dentry->d_flags & DCACHE_NFSFS_RENAMED) ||
WARN_ON(new_dentry->d_fsdata == NFS_FSDATA_BLOCKED))
goto out;
if (new_dentry->d_fsdata) {
/* old devname */
kfree(new_dentry->d_fsdata);
new_dentry->d_fsdata = NULL;
}
spin_lock(&new_dentry->d_lock);
if (d_count(new_dentry) > 2) {
@ -2755,7 +2770,7 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
new_dentry = dentry;
new_inode = NULL;
} else {
new_dentry->d_fsdata = NFS_FSDATA_BLOCKED;
block_revalidate(new_dentry);
must_unblock = true;
spin_unlock(&new_dentry->d_lock);
}
@ -2767,6 +2782,8 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
task = nfs_async_rename(old_dir, new_dir, old_dentry, new_dentry,
must_unblock ? nfs_unblock_rename : NULL);
if (IS_ERR(task)) {
if (must_unblock)
unblock_revalidate(new_dentry);
error = PTR_ERR(task);
goto out;
}