From: NeilBrown <[email protected]> It is important that two non-create NFS "open"s of a negative dentry don't race. They both have only a shared lock on i_rwsem and so could run concurrently, but they might both try to call d_splice_alias() at the same time which is confusing at best.
nfs_atomic_open() currently avoids this by discarding the negative dentry and creating a new one using d_alloc_parallel(). Only one thread can successfully get the d_in_lookup() dentry, the other will wait for the first to finish, and can use the result of that first lookup. A proposed locking change inverts the order between i_rwsem and d_alloc_parallel() so it will not be safe to call d_alloc_parallel() while holding i_rwsem - even shared. We can achieve the same effect by causing ->d_revalidate to invalidate a negative dentry when LOOKUP_OPEN is set. Doing this is consistent with the "close to open" caching semantics of NFS which requires the server to be queried whenever opening a file - cached information must not be trusted. With this change to ->d_revaliate (implemented in nfs_neg_need_reval) we can be sure that we have exclusive access to any dentry that reaches nfs_atomic_open(). Either O_CREAT was requested and so the parent is locked exclusively, or the dentry will have DCACHE_PAR_LOOKUP set. This means that the d_drop() and d_alloc_parallel() calls in nfs_atomic_lookup() are no longer needed to provide exclusion Signed-off-by: NeilBrown <[email protected]> --- fs/nfs/dir.c | 30 +++++++----------------------- 1 file changed, 7 insertions(+), 23 deletions(-) diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c index 52e7656195ec..3033cc5ce12f 100644 --- a/fs/nfs/dir.c +++ b/fs/nfs/dir.c @@ -1657,6 +1657,13 @@ int nfs_neg_need_reval(struct inode *dir, struct dentry *dentry, { if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) return 0; + if (flags & LOOKUP_OPEN) + /* close-to-open semantics require we go to server + * on each open. By invalidating the dentry we + * also ensure nfs_atomic_open() always has exclusive + * access to the dentry. + */ + return 0; if (NFS_SERVER(dir)->flags & NFS_MOUNT_LOOKUP_CACHE_NONEG) return 1; /* Case insensitive server? Revalidate negative dentries */ @@ -2112,7 +2119,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry, struct inode *inode; unsigned int lookup_flags = 0; unsigned long dir_verifier; - bool switched = false; int created = 0; int err; @@ -2157,17 +2163,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry, attr.ia_size = 0; } - if (!(open_flags & O_CREAT) && !d_in_lookup(dentry)) { - d_drop(dentry); - switched = true; - dentry = d_alloc_parallel(dentry->d_parent, - &dentry->d_name); - if (IS_ERR(dentry)) - return PTR_ERR(dentry); - if (unlikely(!d_in_lookup(dentry))) - return finish_no_open(file, dentry); - } - ctx = create_nfs_open_context(dentry, open_flags, file); err = PTR_ERR(ctx); if (IS_ERR(ctx)) @@ -2210,10 +2205,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry, trace_nfs_atomic_open_exit(dir, ctx, open_flags, err); put_nfs_open_context(ctx); out: - if (unlikely(switched)) { - d_lookup_done(dentry); - dput(dentry); - } return err; no_open: @@ -2236,13 +2227,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry, res = ERR_PTR(-EOPENSTALE); } } - if (switched) { - d_lookup_done(dentry); - if (!res) - res = dentry; - else - dput(dentry); - } return finish_no_open(file, res); } EXPORT_SYMBOL_GPL(nfs_atomic_open); -- 2.50.0.107.gf914562f5916.dirty
