Bug #129
closednamecache coherency 3rd turn
0%
Description
Hi!
This time the following changes were made:
- Namecache locks are kept by shadowinfo structures which
can be embedded into the namecache structure itself (singleton
groups) or fetched from a pool (for non-singleton groups).
- Shadow group data structure: I ended up at what has been
suggested by Matt earlier: shadow group entries form a circular list.
Double linked in order to support O(1) node deletion, enhanced
with a height counter to retain tree semantics.
- cache_setunresolved(ncp) blows up subtree over ncp to break broken
topologies.
- Deadlock avoiding techniques of the previous patch have been kept.
- The "struct namecache *nc_shadowed" field of namecache structures
still exists, but is nowhere referred by cache code. Now it's sort
of a private field, almost like the "void *" fields in vnodes,
specinfo structures, etc. It could be easily ditched and replaced by
per-mount hash for those fs-es who would use it. I just kept it as
is -- I didn't want to do anything about it without having a
consensus.
- Nullfs adjusted to this API.
Regards,
Csaba
Files
Updated by csaba.henk over 18 years ago
Minor update:
- made cache code topology agnostic (ie., tree layout hints [height] are
maintained but not used by the cache code itself, group members are
treated uniformly);
- made group traversal routine public;
- panic fix for nullfs.
The respective diffs for the cache system and nullfs are available here:
http://creo.hu/~csaba/tmp/visible/dfly/
as well as incremental diffs from the latest posted patches.
Regards,
Csaba
Updated by corecode over 18 years ago
On 03.04.2006, at 09:51, Csaba Henk wrote:
static struct shadowinfo *
+shadowinfo_fetch(void)
{
+ struct shadowinfo *shinf = STAILQ_FIRST(&shadowinfo_freeq);
if (! shinf)
+ goto alloc;
I'm strongly against adding a cache mechanism here. We should use a
generic object cache whereever possible (and bring that one in shape,
hint, hint). Then this would be a matter of:
shinf = objcache_alloc(shadowinfo_cache, M_WAITOK);
the approach you are taking is not MP-friendly at all, as it needs a
lock, but I think you aware of this anyways. Just let us not do such
optimizations prematurely. using malloc() should be sufficient right
now.
@ -295,6 +360,10
@ cache_alloc(int nlen)
ncp->nc_error = ENOTCONN; /* needs to be resolved */
ncp->nc_refs = 1;
ncp->nc_fsmid = 1;
+ ncp->nc_shadowinfo = &ncp->nc_shadowinfo_internal;
+ ncp->nc_shadowinfo_internal.sh_refs = 2;
+ ncp->nc_shadow_prev = NULL;
+ ncp->nc_shadow_next = NULL;
why is refs 2? This would at least need a comment explaining this
number.
sncp is locked here:
[1]:
int
+cache_shadow_attach(struct namecache *ncp, struct namecache *sncp)
{
+ struct migrate_param mpm;
[..]
+ if (sncp->nc_shadowinfo &sncp->nc_shadowinfo_internal) {
+ mpm.heightdelta = 0;
+ mpm.shadowinfo = shadowinfo_fetch();
+ mpm.exlocks = sncp->nc_shadowinfo->sh_exlocks;
+ migrate_updater(sncp, &mpm);
+ }
[2]:
[..]
@ -372,16 +628,15
@ cache_lock(struct namecache *ncp)
[..]
- ncp->nc_flag |= NCF_LOCKREQ;
- if (tsleep(ncp, 0, "clock", nclockwarn) == EWOULDBLOCK) {
+ shinf->sh_lockreq = 1;
+ if (tsleep(shinf, 0, "clock", nclockwarn) EWOULDBLOCK) {
[..]
[..]
+ if (--shinf->sh_exlocks 0) {
+ shinf->sh_locktd = NULL;
+ if (shinf->sh_lockreq) {
+ shinf->sh_lockreq = 0;
+ wakeup(shinf);
I think there is a race condition which Matt already pointed out:
Thread1 Thread2
[2]
[2] (tsleeps on ncp->nc_shadowinfo_internal)
[1] (migrates shinfo to an external struct)
[3] (wakes up external shinfo)
ncp is unlocked and gets freed
thread2 will sleep on ncp, but will never get woken up. tsleep will
return after some time and then access a non-existing ncp.
cheers
simon
Updated by csaba.henk over 18 years ago
On Mon, Apr 03, 2006 at 01:24:50PM +0200, Simon 'corecode' Schubert wrote:
I updated the patch according to your suggestions (plus fixed a panic). See
again
http://creo.hu/~csaba/tmp/visible/dfly/ ,
updated.
generic object cache whereever possible (and bring that one in shape,
hint, hint). Then this would be a matter of:shinf = objcache_alloc(shadowinfo_cache, M_WAITOK);
What do you think of? In what ways would this be more than a
macro-glorified version of the simplistic design I had for shadowinfo?
Would it use low-level optimized allocation routines?
Just let us not do such optimizations prematurely. using malloc()
should be sufficient right now.
Well OK, that just made the code simpler ATM.
@ -295,6 +360,10
@ cache_alloc(int nlen)
ncp->nc_error = ENOTCONN; /* needs to be resolved */
ncp->nc_refs = 1;
ncp->nc_fsmid = 1;
+ ncp->nc_shadowinfo = &ncp->nc_shadowinfo_internal;
+ ncp->nc_shadowinfo_internal.sh_refs = 2;
+ ncp->nc_shadow_prev = NULL;
+ ncp->nc_shadow_next = NULL;why is refs 2? This would at least need a comment explaining this
number.
Sure, I added:
/*
* nc_shadowinfo_internal:
* one ref for being contained in ncp, one for ncp being locked via it.
* (This leverages us from dealing with "internal" vs. "standalone"
* when dealing with shadowinfo references).
*/
Alternatively, there could be a flag displaying if the shadowinfo is
internal or standalone, and make the ref/put routines act based on this.
Would that be preferable?
sncp is locked here:
[1]:int
+cache_shadow_attach(struct namecache *ncp, struct namecache *sncp)
{
+ struct migrate_param mpm;[..]
+ if (sncp->nc_shadowinfo &sncp->nc_shadowinfo_internal) {
+ mpm.heightdelta = 0;
+ mpm.shadowinfo = shadowinfo_fetch();
+ mpm.exlocks = sncp->nc_shadowinfo->sh_exlocks;
+ migrate_updater(sncp, &mpm);
+ }[2]:
[..]
@ -372,16 +628,15
@ cache_lock(struct namecache *ncp)[..]
- ncp->nc_flag |= NCF_LOCKREQ;
- if (tsleep(ncp, 0, "clock", nclockwarn) == EWOULDBLOCK) {
+ shinf->sh_lockreq = 1;
+ if (tsleep(shinf, 0, "clock", nclockwarn) EWOULDBLOCK) {[..]
[3]:
@ -436,17 +695,45
@ cache_unlock(struct namecache *ncp)
cache_unlock(struct namecache *ncp) {[..]
+ if (--shinf->sh_exlocks 0) {
+ shinf->sh_locktd = NULL;
+ if (shinf->sh_lockreq) {
+ shinf->sh_lockreq = 0;
+ wakeup(shinf);I think there is a race condition which Matt already pointed out:
Thread1 Thread2
[2][2] (tsleeps on ncp->nc_shadowinfo_internal)
[1] (migrates shinfo to an external struct)
[3] (wakes up external shinfo)
ncp is unlocked and gets freedthread2 will sleep on ncp, but will never get woken up. tsleep will
return after some time and then access a non-existing ncp.
Right -- I just forgot about it... Now I simply inserted a wakeup() before
putting the shadowinfo in the migration routine.
AFAICS that's enough and we don't need to hold an extra ref for the
shadowinfo while sleeping on it in cache_lock(), because the variable
"shinf" gets re-initialized when sleeping is over (to the same value or
to something else -- that doesn't make a difference for the lock
routine).
Regards,
Csaba
Updated by alexh about 15 years ago
There has been a lot of work on the namecache since this was posted; is it
still of interest?
Cheers,
Alex Hornung
Updated by csaba.henk about 15 years ago
I think these things have been dealt with since then. (Pls confirm someone who is
more involved than me in Dfly...(O
Csaba
Updated by ftigeot almost 13 years ago
- Description updated (diff)
- Status changed from New to Closed
- Assignee deleted (
0)
Closing due to lack of recent feedback.