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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: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251114201803.2183505-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Mateusz Guzik 2025-11-14 21:18:03 +01:00 committed by Christian Brauner
parent 6d228c181e
commit bfef6e1f34
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 22 additions and 16 deletions
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38
fs/namespace.c
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@ -1345,26 +1345,12 @@ static void delayed_mntput(struct work_struct *unused)
} }
static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput); 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); LIST_HEAD(list);
int count; int count;
rcu_read_lock(); VFS_BUG_ON(mnt->mnt_ns);
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;
}
lock_mount_hash(); lock_mount_hash();
/* /*
* make sure that if __legitimize_mnt() has not seen us grab * 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); 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) void mntput(struct vfsmount *mnt)
{ {
if (mnt) { if (mnt) {