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

[Serge E. Hallyn]

Quoting Matt Helsley (matthltc at us.ibm.com):
> 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 <matthltc at us.ibm.com>
> Cc: David Howells <dhowells at redhat.com>
> Cc: Pavel Emelyanov <xemul at parallels.com>
> Cc: Alexey Dobriyan <adobriyan at gmail.com>
> Cc: "Serge E. Hallyn" <serge at hallyn.com>

Acked-by: Serge E. Hallyn <serge at hallyn.com>

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

> Cc: containers at lists.linux-foundation.org
> ---
>  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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