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vfs: save on atomics on the root vnode for absolute lookups

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There are 2 back-to-back atomics on the vnode, but we can check upfront if one
is sufficient. Similarly we can handle relative lookups where current working
directory == root directory.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D23427
This commit is contained in:
mjg 2020-02-01 06:40:35 +00:00
parent 2e8e2366c0
commit e4d3a982d6
1 changed files with 23 additions and 6 deletions
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29
sys/kern/vfs_lookup.c
View File

@ -254,7 +254,7 @@ namei_cleanup_cnp(struct componentname *cnp)
}
static int
namei_handle_root(struct nameidata *ndp, struct vnode **dpp)
namei_handle_root(struct nameidata *ndp, struct vnode **dpp, u_int n)
{
struct componentname *cnp;
@ -276,7 +276,7 @@ namei_handle_root(struct nameidata *ndp, struct vnode **dpp)
ndp->ni_pathlen--;
}
*dpp = ndp->ni_rootdir;
vrefact(*dpp);
vrefactn(*dpp, n);
return (0);
}
@ -395,8 +395,11 @@ namei(struct nameidata *ndp)
* Get starting point for the translation.
*/
FILEDESC_SLOCK(fdp);
/*
* The reference on ni_rootdir is acquired in the block below to avoid
* back-to-back atomics for absolute lookups.
*/
ndp->ni_rootdir = fdp->fd_rdir;
vrefact(ndp->ni_rootdir);
ndp->ni_topdir = fdp->fd_jdir;
/*
@ -412,15 +415,29 @@ namei(struct nameidata *ndp)
cnp->cn_nameptr = cnp->cn_pnbuf;
if (cnp->cn_pnbuf[0] == '/') {
ndp->ni_resflags |= NIRES_ABS;
error = namei_handle_root(ndp, &dp);
error = namei_handle_root(ndp, &dp, 2);
if (error != 0) {
/*
* Simplify error handling, we should almost never be
* here.
*/
vrefact(ndp->ni_rootdir);
}
} else {
if (ndp->ni_startdir != NULL) {
vrefact(ndp->ni_rootdir);
dp = ndp->ni_startdir;
startdir_used = 1;
} else if (ndp->ni_dirfd == AT_FDCWD) {
dp = fdp->fd_cdir;
vrefact(dp);
if (dp == ndp->ni_rootdir) {
vrefactn(dp, 2);
} else {
vrefact(ndp->ni_rootdir);
vrefact(dp);
}
} else {
vrefact(ndp->ni_rootdir);
rights = ndp->ni_rightsneeded;
cap_rights_set(&rights, CAP_LOOKUP);
@ -567,7 +584,7 @@ namei(struct nameidata *ndp)
cnp->cn_nameptr = cnp->cn_pnbuf;
if (*(cnp->cn_nameptr) == '/') {
vrele(dp);
error = namei_handle_root(ndp, &dp);
error = namei_handle_root(ndp, &dp, 1);
if (error != 0)
goto out;
}