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memfs_ll: lock rename's two directories in canonical inode order

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memfs_rename locked the source then destination directory positionally.
Two concurrent renames in opposite directions could therefore deadlock
if the global inodes_mutex were ever relaxed to allow them to run
in parallel.  Today the exclusive Inodes.lock() held for the whole
operation serializes them, so the deadlock is masked, but the positional
order is a latent hazard.

Order the two directory locks by inode number -- always lock the lower
ino first -- so any two renames acquire the per-directory locks in the
same order. The ordering locals are declared with the other rename
locals, before the first goto, because the validation-failure goto
would otherwise cross an initialized declaration into its scope. The
operation body still acts on the directories by role; both are locked
regardless of which was taken first.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
This commit is contained in:
Bernd Schubert
2026-06-17 11:59:46 +02:00
parent b137cc849e
commit 7952b4b69d
1 changed files with 12 additions and 8 deletions
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example/memfs_ll.cc
+12 -8
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@@ -920,6 +920,8 @@ static void memfs_rename(fuse_req_t req, fuse_ino_t parent, const char *name,
Dentry *child_dentry = nullptr;
Dentry *child_dentry_copy = nullptr;
Dentry *existing_dentry = nullptr;
Inode *inode_a = nullptr;
Inode *inode_b = nullptr;
#if defined(RENAME_EXCHANGE) && defined(RENAME_NOREPLACE)
if (flags & (RENAME_EXCHANGE | RENAME_NOREPLACE)) {
@@ -940,10 +942,13 @@ static void memfs_rename(fuse_req_t req, fuse_ino_t parent, const char *name,
goto out_unlock_global;
}
parentInode->lock();
if (parent != newparent) {
newparentInode->lock();
}
inode_a = parentInode.get();
inode_b = (parent != newparent) ? newparentInode.get() : nullptr;
if (inode_b && inode_a->get_ino() > inode_b->get_ino())
std::swap(inode_a, inode_b); // always lock lower ino first
inode_a->lock();
if (inode_b)
inode_b->lock();
child_dentry = parentInode->find_child_locked(name);
if (child_dentry == nullptr) {
@@ -969,10 +974,9 @@ static void memfs_rename(fuse_req_t req, fuse_ino_t parent, const char *name,
newparentInode->add_child_locked(newname, child_dentry_copy);
out_unlock:
parentInode->unlock();
if (parent != newparent) {
newparentInode->unlock();
}
if (inode_b)
inode_b->unlock();
inode_a->unlock();
out_unlock_global:
Inodes.unlock();