Re: [RFC][PATCH] (#7U5) [2-6.12-rc3-mm3] file system auditing
On Wednesday 11 May 2005 14:51, serue(a)us.ibm.com wrote:
Quoting Timothy R. Chavez (tinytim(a)us.ibm.com):
> On Wednesday 11 May 2005 14:01, serue(a)us.ibm.com wrote:
> > Quoting Timothy R. Chavez (tinytim(a)us.ibm.com):
> > > * I've completely removed the audit_master_watchlist_lock spinlock to
> > > protect the master watchlist, in favor of RCU locking. ?To protect
> > > against concurrent watch removals from the master watchlist, we
> > > conveniently use the local data->lock rw_lock in place of another
> > > spinlock. ?Thus, only one removal can
> >
> > I have a problem with this. Isn't the audit_master_watchlist a global
> > list? Are you sure you can use a per-inode lock to protect this global
> > list?
>
> Well, my conclusion was this: The only way to enter
> audit_destroy_wentry() where contention is a concern is by holding the
> local data->lock. The only way we can remove a watch from the master
> watchlist is by entering the audit_destroy_wentry() function (and if
> contentious, only one of the contenders may be in audit_destroy_wentry()
> at a time).
>
> I don't see how we could race on a master watchlist deletion.
1. What if you end up trying to delete two wentries which are adjecent
on the audit_master_watchlist concurrently? I think you might be able
to break the list, but I haven't drawn it down on paper...
Hm. Yeah, sounds like in this case, it'll break. I'll have to think about
this scenario a bit more.
2. What about additions to the audit_master_watchlist, both racing with
each other and with deletions? Actually it doesn't look like you're
protecting additions at all.
There won't be racing insertions. The audit_netlink_sem ensures us that there
will only ever be one insertion at a time and while we're inserting. Also,
if we are inserting, we can't explicitly be removing. The only way to delete
is from audit_update_watch(). So conceptually, I suppose, if we're in the
process of draining the watchlist and we're attempting to insert into that
watchlist at the same time, there might be some ill effect? I suppose it
doesn't hurt to keep the locking here for this case.
-serge
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-tim