2011-02-18 00:38:21

by Matt Helsley

Subject: [PATCH] Reduce uidhash lock hold time when lookup succeeds

When lookup succeeds we don't need the "new" user struct which hasn't
been linked into the uidhash. So we can immediately drop the lock and
then free "new" rather than free it with the lock held.

Signed-off-by: Matt Helsley <[email protected]>
Cc: David Howells <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: "Serge E. Hallyn" <[email protected]>
Cc: [email protected]
---
kernel/user.c | 12 +++++++-----
1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/user.c b/kernel/user.c
index 5c598ca..4ea8e58 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -157,16 +157,18 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
*/
spin_lock_irq(&uidhash_lock);
up = uid_hash_find(uid, hashent);
- if (up) {
+ if (!up) {
+ uid_hash_insert(new, hashent);
+ up = new;
+ }
+ spin_unlock_irq(&uidhash_lock);
+
+ if (up != new) {
put_user_ns(ns);
key_put(new->uid_keyring);
key_put(new->session_keyring);
kmem_cache_free(uid_cachep, new);
- } else {
- uid_hash_insert(new, hashent);
- up = new;
}
- spin_unlock_irq(&uidhash_lock);
}

return up;
--
1.6.3.3

2011-02-18 18:25:44

by Serge Hallyn

Subject: Re: [PATCH] Reduce uidhash lock hold time when lookup succeeds

Quoting Matt Helsley ([email protected]):
> When lookup succeeds we don't need the "new" user struct which hasn't
> been linked into the uidhash. So we can immediately drop the lock and
> then free "new" rather than free it with the lock held.
>
> Signed-off-by: Matt Helsley <[email protected]>
> Cc: David Howells <[email protected]>
> Cc: Pavel Emelyanov <[email protected]>
> Cc: Alexey Dobriyan <[email protected]>
> Cc: "Serge E. Hallyn" <[email protected]>

Acked-by: Serge E. Hallyn <[email protected]>

And might I say that the label 'out_unlock' in that function is
sadly named :)

> Cc: [email protected]
> ---
> kernel/user.c | 12 +++++++-----
> 1 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/user.c b/kernel/user.c
> index 5c598ca..4ea8e58 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -157,16 +157,18 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
> */
> spin_lock_irq(&uidhash_lock);
> up = uid_hash_find(uid, hashent);
> - if (up) {
> + if (!up) {
> + uid_hash_insert(new, hashent);
> + up = new;
> + }
> + spin_unlock_irq(&uidhash_lock);
> +
> + if (up != new) {
> put_user_ns(ns);
> key_put(new->uid_keyring);
> key_put(new->session_keyring);
> kmem_cache_free(uid_cachep, new);
> - } else {
> - uid_hash_insert(new, hashent);
> - up = new;
> }
> - spin_unlock_irq(&uidhash_lock);
> }
>
> return up;
> --
> 1.6.3.3
>
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