Module Name: src
Committed By: dennis
Date: Sun Dec 7 22:23:38 UTC 2014
Modified Files:
src/sys/kern: vfs_cache.c
Log Message:
Take 2. Do a fairly mechanical reversion of the locking protocol
to that of revision 1.94. Add a comment documenting my best guess
about the locking requirements in this subsystem. Don't take locks
solely for the sake of stats counter increments; the locking around
the increment of ->ncs_pass2 and/or ->ncs_2passes caused a deadlock.
Defer fixing stats-keeping to allow this change to be easily backed
out of.
To generate a diff of this commit:
cvs rdiff -u -r1.101 -r1.102 src/sys/kern/vfs_cache.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/vfs_cache.c
diff -u src/sys/kern/vfs_cache.c:1.101 src/sys/kern/vfs_cache.c:1.102
--- src/sys/kern/vfs_cache.c:1.101 Wed Dec 3 01:30:32 2014
+++ src/sys/kern/vfs_cache.c Sun Dec 7 22:23:38 2014
@@ -1,4 +1,4 @@
-/* $NetBSD: vfs_cache.c,v 1.101 2014/12/03 01:30:32 christos Exp $ */
+/* $NetBSD: vfs_cache.c,v 1.102 2014/12/07 22:23:38 dennis Exp $ */
/*-
* Copyright (c) 2008 The NetBSD Foundation, Inc.
@@ -58,7 +58,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_cache.c,v 1.101 2014/12/03 01:30:32 christos Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_cache.c,v 1.102 2014/12/07 22:23:38 dennis Exp $");
#include "opt_ddb.h"
#include "opt_revcache.h"
@@ -102,7 +102,39 @@ __KERNEL_RCSID(0, "$NetBSD: vfs_cache.c,
*/
/*
- * Per-cpu namecache data.
+ * The locking in this subsystem works as follows:
+ *
+ * When an entry is added to the cache, via cache_enter(),
+ * namecache_lock is taken to exclude other writers. The new
+ * entry is added to the hash list in a way which permits
+ * concurrent lookups and invalidations in the cache done on
+ * other CPUs to continue in parallel.
+ *
+ * When a lookup is done in the cache, via cache_lookup() or
+ * cache_lookup_raw(), the per-cpu lock below is taken. This
+ * protects calls to cache_lookup_entry() and cache_invalidate()
+ * against cache_reclaim() but allows lookups to continue in
+ * parallel with cache_enter().
+ *
+ * cache_revlookup() takes namecache_lock to exclude cache_enter()
+ * and cache_reclaim() since the list it operates on is not
+ * maintained to allow concurrent reads.
+ *
+ * When cache_reclaim() is called namecache_lock is held to hold
+ * off calls to cache_enter()/cache_revlookup() and each of the
+ * per-cpu locks is taken to hold off lookups. Holding all these
+ * locks essentially idles the subsystem, ensuring there are no
+ * concurrent references to the cache entries being freed.
+ *
+ * As a side effect while running cache_reclaim(), once the per-cpu
+ * locks are held the per-cpu stats are sampled, added to the
+ * subsystem total and zeroed. Only some of the per-cpu stats are
+ * incremented with the per-cpu lock held, however, and attempting
+ * to add locking around the remaining counters currently
+ * incremented without a lock can cause deadlock, so we don't
+ * do that. XXX Fix this up in a later revision.
+ *
+ * Per-cpu namecache data is defined next.
*/
struct nchcpu {
kmutex_t cpu_lock;
@@ -286,7 +318,8 @@ cache_unlock_cpus(void)
}
/*
- * Find a single cache entry and return it locked.
+ * Find a single cache entry and return it locked. 'namecache_lock' or
+ * at least one of the per-CPU locks must be held.
*/
static struct namecache *
cache_lookup_entry(const struct vnode *dvp, const char *name, size_t namelen)
@@ -296,7 +329,6 @@ cache_lookup_entry(const struct vnode *d
nchash_t hash;
KASSERT(dvp != NULL);
- KASSERT(mutex_owned(namecache_lock));
hash = cache_hash(name, namelen);
ncpp = &nchashtbl[NCHASH2(hash, dvp)];
@@ -388,27 +420,22 @@ cache_lookup(struct vnode *dvp, const ch
}
cpup = curcpu()->ci_data.cpu_nch;
+ mutex_enter(&cpup->cpu_lock);
if (__predict_false(namelen > NCHNAMLEN)) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_long);
mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
- mutex_enter(namecache_lock);
ncp = cache_lookup_entry(dvp, name, namelen);
- mutex_exit(namecache_lock);
if (__predict_false(ncp == NULL)) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_miss);
mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
if ((cnflags & MAKEENTRY) == 0) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_badhits);
- mutex_exit(&cpup->cpu_lock);
/*
* Last component and we are renaming or deleting,
* the cache entry is invalid, or otherwise don't
@@ -416,6 +443,7 @@ cache_lookup(struct vnode *dvp, const ch
*/
cache_invalidate(ncp);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
@@ -432,16 +460,13 @@ cache_lookup(struct vnode *dvp, const ch
if (__predict_true(nameiop != CREATE ||
(cnflags & ISLASTCN) == 0)) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_neghits);
- mutex_exit(&cpup->cpu_lock);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
/* found neg entry; vn is already null from above */
return 1;
} else {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_badhits);
- mutex_exit(&cpup->cpu_lock);
/*
* Last component and we are renaming or
* deleting, the cache entry is invalid,
@@ -450,6 +475,7 @@ cache_lookup(struct vnode *dvp, const ch
*/
cache_invalidate(ncp);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
@@ -458,6 +484,7 @@ cache_lookup(struct vnode *dvp, const ch
vp = ncp->nc_vp;
mutex_enter(vp->v_interlock);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
error = vget(vp, LK_NOWAIT);
if (error) {
KASSERT(error == EBUSY);
@@ -465,9 +492,7 @@ cache_lookup(struct vnode *dvp, const ch
* This vnode is being cleaned out.
* XXX badhits?
*/
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_falsehits);
- mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
@@ -481,9 +506,7 @@ cache_lookup(struct vnode *dvp, const ch
#endif /* DEBUG */
/* We don't have the right lock, but this is only for stats. */
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_goodhits);
- mutex_exit(&cpup->cpu_lock);
/* found it */
*vn_ret = vp;
@@ -512,18 +535,15 @@ cache_lookup_raw(struct vnode *dvp, cons
}
cpup = curcpu()->ci_data.cpu_nch;
+ mutex_enter(&cpup->cpu_lock);
if (__predict_false(namelen > NCHNAMLEN)) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_long);
mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
- mutex_enter(namecache_lock);
ncp = cache_lookup_entry(dvp, name, namelen);
- mutex_exit(namecache_lock);
if (__predict_false(ncp == NULL)) {
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_miss);
mutex_exit(&cpup->cpu_lock);
/* found nothing */
@@ -539,15 +559,15 @@ cache_lookup_raw(struct vnode *dvp, cons
/*cnp->cn_flags |= ncp->nc_flags;*/
*iswht_ret = (ncp->nc_flags & ISWHITEOUT) != 0;
}
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_neghits);
- mutex_exit(&cpup->cpu_lock);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
/* found negative entry; vn is already null from above */
return 1;
}
mutex_enter(vp->v_interlock);
mutex_exit(&ncp->nc_lock);
+ mutex_exit(&cpup->cpu_lock);
error = vget(vp, LK_NOWAIT);
if (error) {
KASSERT(error == EBUSY);
@@ -555,17 +575,13 @@ cache_lookup_raw(struct vnode *dvp, cons
* This vnode is being cleaned out.
* XXX badhits?
*/
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_falsehits);
- mutex_exit(&cpup->cpu_lock);
/* found nothing */
return 0;
}
/* Unlocked, but only for stats. */
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_goodhits); /* XXX can be "badhits" */
- mutex_exit(&cpup->cpu_lock);
/* found it */
*vn_ret = vp;
@@ -617,9 +633,7 @@ cache_revlookup(struct vnode *vp, struct
ncp->nc_name[1] == '.')
panic("cache_revlookup: found entry for ..");
#endif
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_revhits);
- mutex_exit(&cpup->cpu_lock);
nlen = ncp->nc_nlen;
if (bufp) {
@@ -651,9 +665,7 @@ cache_revlookup(struct vnode *vp, struct
}
mutex_exit(&ncp->nc_lock);
}
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_revmiss);
- mutex_exit(&cpup->cpu_lock);
mutex_exit(namecache_lock);
out:
*dvpp = NULL;
@@ -1101,9 +1113,7 @@ namecache_count_pass2(void)
{
struct nchcpu *cpup = curcpu()->ci_data.cpu_nch;
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_pass2);
- mutex_exit(&cpup->cpu_lock);
}
void
@@ -1111,9 +1121,7 @@ namecache_count_2passes(void)
{
struct nchcpu *cpup = curcpu()->ci_data.cpu_nch;
- mutex_enter(&cpup->cpu_lock);
COUNT(cpup->cpu_stats, ncs_2passes);
- mutex_exit(&cpup->cpu_lock);
}
static int
