> On 30 Dec 2014, at 21:40 , Mateusz Guzik <m...@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."
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