Unverified Commit bfef6e1f authored by Mateusz Guzik's avatar Mateusz Guzik Committed by Christian Brauner
Browse files

fs: move mntput_no_expire() slowpath into a dedicated routine 



In the stock variant the compiler spills several registers on the stack
and employs stack smashing protection, adding even more code + a branch
on exit..

The actual fast path is small enough that the compiler inlines it for
all callers -- the symbol is no longer emitted.

Forcing noinline on it just for code-measurement purposes shows the fast
path dropping from 111 to 39 bytes.

Signed-off-by: default avatarMateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251114201803.2183505-1-mjguzik@gmail.com


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 6d228c18

fs/namespace.c

+22 −16
Original line number Diff line number Diff line
@@ -1345,26 +1345,12 @@ static void delayed_mntput(struct work_struct *unused) 
}

static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);



static void mntput_no_expire(struct mount *mnt)

static void noinline mntput_no_expire_slowpath(struct mount *mnt)

{

	LIST_HEAD(list);

	int count;



	rcu_read_lock();

	if (likely(READ_ONCE(mnt->mnt_ns))) {

		/*

		 * Since we don't do lock_mount_hash() here,

		 * ->mnt_ns can change under us.  However, if it's

		 * non-NULL, then there's a reference that won't

		 * be dropped until after an RCU delay done after

		 * turning ->mnt_ns NULL.  So if we observe it

		 * non-NULL under rcu_read_lock(), the reference

		 * we are dropping is not the final one.

		 */

		mnt_add_count(mnt, -1);

		rcu_read_unlock();

		return;

	}

	VFS_BUG_ON(mnt->mnt_ns);

	lock_mount_hash();

	/*

	 * make sure that if __legitimize_mnt() has not seen us grab
@@ -1415,6 +1401,26 @@ static void mntput_no_expire(struct mount *mnt) 
	cleanup_mnt(mnt);

}



static void mntput_no_expire(struct mount *mnt)

{

	rcu_read_lock();

	if (likely(READ_ONCE(mnt->mnt_ns))) {

		/*

		 * Since we don't do lock_mount_hash() here,

		 * ->mnt_ns can change under us.  However, if it's

		 * non-NULL, then there's a reference that won't

		 * be dropped until after an RCU delay done after

		 * turning ->mnt_ns NULL.  So if we observe it

		 * non-NULL under rcu_read_lock(), the reference

		 * we are dropping is not the final one.

		 */

		mnt_add_count(mnt, -1);

		rcu_read_unlock();

		return;

	}

	mntput_no_expire_slowpath(mnt);

}



void mntput(struct vfsmount *mnt)

{

	if (mnt) {