Re: [PATCH v3 05/19] VFS: introduce d_alloc_noblock()

On Tue, 28 Apr 2026, Al Viro wrote:
> On Mon, Apr 27, 2026 at 02:01:23PM +1000, NeilBrown wrote:
> > From: NeilBrown <[email protected]>
> > 
> > Several filesystems use the results of readdir to prime the dcache.
> > These filesystems use d_alloc_parallel() which can block if there is a
> > concurrent lookup.  Blocking in that case is pointless as the lookup
> > will add info to the dcache and there is no value in the readdir waiting
> > to see if it should add the info too.
> 
> ... except that there is - large part of the reasons for that in the original
> user (procfs) is that we want getdents() + open() + fstat() + compare ->st_ino
> from fstat() with ->d_ino from getdents() to work, even if you race with lookup
> from another process coming in the middle of your getdents().
> 
> What are your plans in that area?
> 

I can't see that procfs needs the directory to be locked at all.
So we could drop the parent lock at the start of readdir and
reclaim at the end.  But I decided to just drop it in the rare case
where it could cause a problem.

Below is that patch I have to procfs.

Thanks,
NeilBrown


Author: NeilBrown <[email protected]>
Date:   Mon Mar 16 16:37:52 2026 +1100

    procfs: drop parent lock for d_alloc_parallel() in iterate_shared()
    
    When procfs finds a name in iterate_shared() that isn't in the dcache it
    *must* add it so that it can have a stable inode number to report
    (inodes are only accessible from the dcache in procfs).
    
    It uses d_alloc_parallel().  A planned change to locking will make it
    unsafe to call d_alloc_parallel() while holding the directory lock.
    
    Other filesystems which prime the dcache in iterate_shared() use
    d_alloc_noblock() which is safe but can fail if it races with ->lookup.
    As procfs cannot handle failure we need something better.
    
    procfs doesn't *need* the parent to be locked.  There are no shared data
    structures accessed that don't have their own locking.  So it is safe to
    drop and re-take the parent lock.  We could do this around the whole
    iteration, but as failure of d_alloc_noblock() is rare it is more
    efficient to drop and retake it just around a call of d_alloc_parallel()
    when d_alloc_noblock does fail.
    
    Other code that drops and retakes the lock in iterate_shared needs to
    be careful to check S_DEAD which could be set while the lock is
    dropped.  This is not needed in procfs as the flag is never set.
    
    As d_alloc_noblock() calls try_lookup_noperm(), we can skip that call
    and simplify the code.
    
    Signed-off-by: NeilBrown <[email protected]>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index d55a4b603188..feed586fa736 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2127,24 +2127,32 @@ bool proc_fill_cache(struct file *file, struct dir_context *ctx,
 	unsigned type = DT_UNKNOWN;
 	ino_t ino = 1;
 
-	child = try_lookup_noperm(&qname, dir);
+	child = d_alloc_noblock(dir, &qname);
 	if (IS_ERR(child))
 		goto end_instantiate;
 
-	if (!child) {
+	if (child == ERR_PTR(-EWOULDBLOCK)) {
+		/*
+		 * Need to drop directory lock, which isn't really
+		 * needed here anyway.  As rmdir never happens in procfs
+		 * we don't need to be concerned about S_DEAD being set
+		 * while unlocked.
+		 */
+		inode_unlock_shared(dir->d_inode);
 		child = d_alloc_parallel(dir, &qname);
-		if (IS_ERR(child))
-			goto end_instantiate;
-		if (d_in_lookup(child)) {
-			struct dentry *res;
-			res = instantiate(child, task, ptr);
-			d_lookup_done(child);
-			if (unlikely(res)) {
-				dput(child);
-				child = res;
-				if (IS_ERR(child))
-					goto end_instantiate;
-			}
+		inode_lock_shared(dir->d_inode);
+	}
+	if (IS_ERR(child))
+		goto end_instantiate;
+	if (d_in_lookup(child)) {
+		struct dentry *res;
+		res = instantiate(child, task, ptr);
+		d_lookup_done(child);
+		if (unlikely(res)) {
+			dput(child);
+			child = res;
+			if (IS_ERR(child))
+				goto end_instantiate;
 		}
 	}
 	inode = d_inode(child);
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 04a382178c65..c19e1ccd6bb9 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -686,29 +686,34 @@ static bool proc_sys_fill_cache(struct file *file,
 	ino_t ino = 0;
 	unsigned type = DT_UNKNOWN;
 
-	qname.name = table->procname;
-	qname.len  = strlen(table->procname);
-	qname.hash = full_name_hash(dir, qname.name, qname.len);
-
-	child = d_lookup(dir, &qname);
-	if (!child) {
+	qname = QSTR(table->procname);
+	child = d_alloc_noblock(dir, &qname);
+	if (child == ERR_PTR(-EWOULDBLOCK)) {
+		/*
+		 * Need to drop directory lock, which isn't really
+		 * needed here anyway.  As rmdir never happens in procfs
+		 * we don't need to be concerned about S_DEAD being set
+		 * while unlocked.
+		 */
+		inode_unlock_shared(dir->d_inode);
 		child = d_alloc_parallel(dir, &qname);
-		if (IS_ERR(child))
-			return false;
-		if (d_in_lookup(child)) {
-			struct dentry *res;
-			inode = proc_sys_make_inode(dir->d_sb, head, table);
-			res = d_splice_alias_ops(inode, child,
-						 &proc_sys_dentry_operations);
-			d_lookup_done(child);
-			if (unlikely(res)) {
-				dput(child);
-
-				if (IS_ERR(res))
-					return false;
-
-				child = res;
-			}
+		inode_lock_shared(dir->d_inode);
+	}
+	if (IS_ERR(child))
+		return false;
+	if (d_in_lookup(child)) {
+		struct dentry *res;
+		inode = proc_sys_make_inode(dir->d_sb, head, table);
+		res = d_splice_alias_ops(inode, child,
+					 &proc_sys_dentry_operations);
+		d_lookup_done(child);
+		if (unlikely(res)) {
+			dput(child);
+
+			if (IS_ERR(res))
+				return false;
+
+			child = res;
 		}
 	}
 	inode = d_inode(child);