svn commit: r276424 - head/sys/kern

[Bjoern A. Zeeb]

> On 30 Dec 2014, at 21:40 , Mateusz Guzik <mjg at FreeBSD.org> wrote:
> 
> Author: mjg
> Date: Tue Dec 30 21:40:45 2014
> New Revision: 276424
> URL: https://svnweb.freebsd.org/changeset/base/276424
> 
> Log:
>  Convert vfs hash lock from a mutex to an rwlock.

This is very obvious from the change.

What is not and should be described as well is some of this:

- Why?
- What are the benefits?
- What are the locking implications?
- Does it make a performance difference?  If yes, was it measured?
- …  (you got the idea)



> 
> Modified:
>  head/sys/kern/vfs_hash.c
> 
> Modified: head/sys/kern/vfs_hash.c
> ==============================================================================
> --- head/sys/kern/vfs_hash.c	Tue Dec 30 20:46:01 2014	(r276423)
> +++ head/sys/kern/vfs_hash.c	Tue Dec 30 21:40:45 2014	(r276424)
> @@ -33,6 +33,7 @@ __FBSDID("$FreeBSD$");
> #include <sys/kernel.h>
> #include <sys/malloc.h>
> #include <sys/mount.h>
> +#include <sys/rwlock.h>
> #include <sys/vnode.h>
> 
> static MALLOC_DEFINE(M_VFS_HASH, "vfs_hash", "VFS hash table");
> @@ -40,14 +41,14 @@ static MALLOC_DEFINE(M_VFS_HASH, "vfs_ha
> static LIST_HEAD(vfs_hash_head, vnode)	*vfs_hash_tbl;
> static LIST_HEAD(,vnode)		vfs_hash_side;
> static u_long				vfs_hash_mask;
> -static struct mtx			vfs_hash_mtx;
> +static struct rwlock			vfs_hash_lock;
> 
> static void
> vfs_hashinit(void *dummy __unused)
> {
> 
> 	vfs_hash_tbl = hashinit(desiredvnodes, M_VFS_HASH, &vfs_hash_mask);
> -	mtx_init(&vfs_hash_mtx, "vfs hash", NULL, MTX_DEF);
> +	rw_init(&vfs_hash_lock, "vfs hash");
> 	LIST_INIT(&vfs_hash_side);
> }
> 
> @@ -75,7 +76,7 @@ vfs_hash_get(const struct mount *mp, u_i
> 	int error;
> 
> 	while (1) {
> -		mtx_lock(&vfs_hash_mtx);
> +		rw_rlock(&vfs_hash_lock);
> 		LIST_FOREACH(vp, vfs_hash_bucket(mp, hash), v_hashlist) {
> 			if (vp->v_hash != hash)
> 				continue;
> @@ -84,7 +85,7 @@ vfs_hash_get(const struct mount *mp, u_i
> 			if (fn != NULL && fn(vp, arg))
> 				continue;
> 			VI_LOCK(vp);
> -			mtx_unlock(&vfs_hash_mtx);
> +			rw_runlock(&vfs_hash_lock);
> 			error = vget(vp, flags | LK_INTERLOCK, td);
> 			if (error == ENOENT && (flags & LK_NOWAIT) == 0)
> 				break;
> @@ -94,7 +95,7 @@ vfs_hash_get(const struct mount *mp, u_i
> 			return (0);
> 		}
> 		if (vp == NULL) {
> -			mtx_unlock(&vfs_hash_mtx);
> +			rw_runlock(&vfs_hash_lock);
> 			*vpp = NULL;
> 			return (0);
> 		}
> @@ -105,9 +106,9 @@ void
> vfs_hash_remove(struct vnode *vp)
> {
> 
> -	mtx_lock(&vfs_hash_mtx);
> +	rw_wlock(&vfs_hash_lock);
> 	LIST_REMOVE(vp, v_hashlist);
> -	mtx_unlock(&vfs_hash_mtx);
> +	rw_wunlock(&vfs_hash_lock);
> }
> 
> int
> @@ -118,7 +119,7 @@ vfs_hash_insert(struct vnode *vp, u_int 
> 
> 	*vpp = NULL;
> 	while (1) {
> -		mtx_lock(&vfs_hash_mtx);
> +		rw_wlock(&vfs_hash_lock);
> 		LIST_FOREACH(vp2,
> 		    vfs_hash_bucket(vp->v_mount, hash), v_hashlist) {
> 			if (vp2->v_hash != hash)
> @@ -128,13 +129,13 @@ vfs_hash_insert(struct vnode *vp, u_int 
> 			if (fn != NULL && fn(vp2, arg))
> 				continue;
> 			VI_LOCK(vp2);
> -			mtx_unlock(&vfs_hash_mtx);
> +			rw_wunlock(&vfs_hash_lock);
> 			error = vget(vp2, flags | LK_INTERLOCK, td);
> 			if (error == ENOENT && (flags & LK_NOWAIT) == 0)
> 				break;
> -			mtx_lock(&vfs_hash_mtx);
> +			rw_wlock(&vfs_hash_lock);
> 			LIST_INSERT_HEAD(&vfs_hash_side, vp, v_hashlist);
> -			mtx_unlock(&vfs_hash_mtx);
> +			rw_wunlock(&vfs_hash_lock);
> 			vput(vp);
> 			if (!error)
> 				*vpp = vp2;
> @@ -146,7 +147,7 @@ vfs_hash_insert(struct vnode *vp, u_int 
> 	}
> 	vp->v_hash = hash;
> 	LIST_INSERT_HEAD(vfs_hash_bucket(vp->v_mount, hash), vp, v_hashlist);
> -	mtx_unlock(&vfs_hash_mtx);
> +	rw_wunlock(&vfs_hash_lock);
> 	return (0);
> }
> 
> @@ -154,9 +155,9 @@ void
> vfs_hash_rehash(struct vnode *vp, u_int hash)
> {
> 
> -	mtx_lock(&vfs_hash_mtx);
> +	rw_wlock(&vfs_hash_lock);
> 	LIST_REMOVE(vp, v_hashlist);
> 	LIST_INSERT_HEAD(vfs_hash_bucket(vp->v_mount, hash), vp, v_hashlist);
> 	vp->v_hash = hash;
> -	mtx_unlock(&vfs_hash_mtx);
> +	rw_wunlock(&vfs_hash_lock);
> }
> 

— 
Bjoern A. Zeeb                                  Charles Haddon Spurgeon:
"Friendship is one of the sweetest joys of life.  Many might have failed
 beneath the bitterness of their trial  had they not found a friend."