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tmpfs: add support for lockless symlink lookup

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Reviewed by:	kib (previous version)
Tested by:	pho (previous version)
Differential Revision:	https://reviews.freebsd.org/D27488
This commit is contained in:
Mateusz Guzik 2021-01-23 13:45:01 +00:00
parent c892d60a1d
commit 618029af50
3 changed files with 81 additions and 7 deletions
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8
sys/fs/tmpfs/tmpfs.h
View File

@ -277,7 +277,10 @@ struct tmpfs_node {
/* Valid when tn_type == VLNK. */
/* The link's target, allocated from a string pool. */
char * tn_link; /* (c) */
struct tn_link {
char * tn_link_target; /* (c) */
char tn_link_smr; /* (c) */
} tn_link;
/* Valid when tn_type == VREG. */
struct tn_reg {
@ -300,7 +303,8 @@ LIST_HEAD(tmpfs_node_list, tmpfs_node);
#define tn_rdev tn_spec.tn_rdev
#define tn_dir tn_spec.tn_dir
#define tn_link tn_spec.tn_link
#define tn_link_target tn_spec.tn_link.tn_link_target
#define tn_link_smr tn_spec.tn_link.tn_link_smr
#define tn_reg tn_spec.tn_reg
#define tn_fifo tn_spec.tn_fifo

50
sys/fs/tmpfs/tmpfs_subr.c
View File

@ -252,6 +252,8 @@ tmpfs_alloc_node(struct mount *mp, struct tmpfs_mount *tmp, enum vtype type,
{
struct tmpfs_node *nnode;
vm_object_t obj;
char *symlink;
char symlink_smr;
/* If the root directory of the 'tmp' file system is not yet
* allocated, this must be the request to do it. */
@ -327,9 +329,41 @@ tmpfs_alloc_node(struct mount *mp, struct tmpfs_mount *tmp, enum vtype type,
case VLNK:
MPASS(strlen(target) < MAXPATHLEN);
nnode->tn_size = strlen(target);
nnode->tn_link = malloc(nnode->tn_size, M_TMPFSNAME,
M_WAITOK);
memcpy(nnode->tn_link, target, nnode->tn_size);
symlink = NULL;
if (!tmp->tm_nonc) {
symlink = cache_symlink_alloc(nnode->tn_size + 1, M_WAITOK);
symlink_smr = true;
}
if (symlink == NULL) {
symlink = malloc(nnode->tn_size + 1, M_TMPFSNAME, M_WAITOK);
symlink_smr = false;
}
memcpy(symlink, target, nnode->tn_size + 1);
/*
* Allow safe symlink resolving for lockless lookup.
* tmpfs_fplookup_symlink references this comment.
*
* 1. nnode is not yet visible to the world
* 2. both tn_link_target and tn_link_smr get populated
* 3. release fence publishes their content
* 4. tn_link_target content is immutable until node destruction,
* where the pointer gets set to NULL
* 5. tn_link_smr is never changed once set
*
* As a result it is sufficient to issue load consume on the node
* pointer to also get the above content in a stable manner.
* Worst case tn_link_smr flag may be set to true despite being stale,
* while the target buffer is already cleared out.
*
* TODO: Since there is no load consume primitive provided
* right now, the load is performed with an acquire fence.
*/
atomic_store_ptr((uintptr_t *)&nnode->tn_link_target,
(uintptr_t)symlink);
atomic_store_char((char *)&nnode->tn_link_smr, symlink_smr);
atomic_thread_fence_rel();
break;
case VREG:
@ -382,6 +416,7 @@ tmpfs_free_node_locked(struct tmpfs_mount *tmp, struct tmpfs_node *node,
bool detach)
{
vm_object_t uobj;
char *symlink;
bool last;
TMPFS_MP_ASSERT_LOCKED(tmp);
@ -417,7 +452,14 @@ tmpfs_free_node_locked(struct tmpfs_mount *tmp, struct tmpfs_node *node,
break;
case VLNK:
free(node->tn_link, M_TMPFSNAME);
symlink = node->tn_link_target;
atomic_store_ptr((uintptr_t *)&node->tn_link_target,
(uintptr_t)NULL);
if (atomic_load_char(&node->tn_link_smr)) {
cache_symlink_free(symlink, node->tn_size + 1);
} else {
free(symlink, M_TMPFSNAME);
}
break;
case VREG:

30
sys/fs/tmpfs/tmpfs_vnops.c
View File

@ -1444,13 +1444,40 @@ tmpfs_readlink(struct vop_readlink_args *v)
node = VP_TO_TMPFS_NODE(vp);
error = uiomove(node->tn_link, MIN(node->tn_size, uio->uio_resid),
error = uiomove(node->tn_link_target, MIN(node->tn_size, uio->uio_resid),
uio);
tmpfs_set_accessed(VFS_TO_TMPFS(vp->v_mount), node);
return (error);
}
/*
* VOP_FPLOOKUP_SYMLINK routines are subject to special circumstances, see
* the comment above cache_fplookup for details.
*
* Check tmpfs_alloc_node for tmpfs-specific synchronisation notes.
*/
static int
tmpfs_fplookup_symlink(struct vop_fplookup_symlink_args *v)
{
struct vnode *vp;
struct tmpfs_node *node;
char *symlink;
vp = v->a_vp;
node = VP_TO_TMPFS_NODE_SMR(vp);
atomic_thread_fence_acq();
if (__predict_false(node == NULL))
return (EAGAIN);
if (!atomic_load_char(&node->tn_link_smr))
return (EAGAIN);
symlink = atomic_load_ptr(&node->tn_link_target);
if (symlink == NULL)
return (EAGAIN);
return (cache_symlink_resolve(v->a_fpl, symlink, node->tn_size));
}
static int
tmpfs_inactive(struct vop_inactive_args *v)
{
@ -1807,6 +1834,7 @@ struct vop_vector tmpfs_vnodeop_entries = {
.vop_open = tmpfs_open,
.vop_close = tmpfs_close,
.vop_fplookup_vexec = tmpfs_fplookup_vexec,
.vop_fplookup_symlink = tmpfs_fplookup_symlink,
.vop_access = tmpfs_access,
.vop_stat = tmpfs_stat,
.vop_getattr = tmpfs_getattr,