It's useless in this case, because without it we can know that
the lock is held or not on a next lock acquisition and even more
if LOCKDEBUG is enabled a failure on the acquisition will provide
useful information for debugging while an assertion failure will
provide just the fact that the assertion failed.
llentry timer (of nd6) holds both llentry's lock and softnet_lock.
A caller also holds them and calls callout_halt to wait for the
timer to quit. However we can pass only one lock to callout_halt,
so passing either of them can cause a deadlock. Fix it by avoid
calling callout_halt without holding llentry's lock.
BTW in the first place we cannot pass llentry's lock to callout_halt
because it's a rwlock...
in6_purgeaddr (in6_unlink_ifa) itself unrefernces a prefix entry and calls
nd6_prelist_remove if the counter becomes 0, so callers doesn't need to
handle the reference counting.
Performance-sensitive paths (sending/forwarding packets) call just one
reader lock. This is a trade-off between performance impact vs. the amount
of efforts; if we want to remove the reader lock, we need huge amount of
works including destroying objects with psz/psref in softint, for example.
The reference counter represents the numuber of references from IPv6
addresses to a prefix entry. If all IPv6 addresses assigned to an
interface are purged, all references to a prefix for the interface are
also released. For now nd6_purge is always called after purging all IPv6
addresses, so we can get rid of clearing pr->ndpr_refcnt from nd6_purge
and instead we can assert it's 0 there.
Note that nd6_ifdetach is only called via dom_ifdetach when processing
if_detach where dom_ifdetach is called after pr_purgeif that eventually
calls in6_ifdetach. So in the call path nd6_purge in nd6_ifdetach does
nothing. That said, we should explicitly make it sure to purge all
IPv6 addresses before nd6_purge for future changes (or the case I missed
something). So if_purgeaddrs is added to nd6_ifdetach.
There were two nd6_purge in in6_ifdetach for some reason, but at least now
We don't need extra nd6_purge. Remove it and instead add assertions that
check if surely purged.
The benefits of the change are:
- We can reduce codes
- We can provide the same behavior between drivers
- Where/When if_ipackets is counted up
- Note that some drivers still update packet statistics in their own
way (periodical update)
- Moved bpf_mtap run in softint
- This makes it easy to MP-ify bpf
Proposed on tech-kern and tech-net
See the following descriptions for details.
Proposed on tech-kern and tech-net
Overview
--------
We protect the routing table with a rwock and protect
rtcaches with another rwlock. Each rtentry is protected
from being freed or updated via reference counting and psref.
Global rwlocks
--------------
There are two rwlocks; one for the routing table (rt_lock) and
the other for rtcaches (rtcache_lock). rtcache_lock covers
all existing rtcaches; there may have room for optimizations
(future work).
The locking order is rtcache_lock first and rt_lock is next.
rtentry references
------------------
References to an rtentry is managed with reference counting
and psref. Either of the two mechanisms is used depending on
where a rtentry is obtained. Reference counting is used when
we obtain a rtentry from the routing table directly via
rtalloc1 and rtrequest{,1} while psref is used when we obtain
a rtentry from a rtcache via rtcache_* APIs. In both cases,
a caller can sleep/block with holding an obtained rtentry.
The reasons why we use two different mechanisms are (i) only
using reference counting hurts the performance due to atomic
instructions (rtcache case) (ii) ease of implementation;
applying psref to APIs such rtaloc1 and rtrequest{,1} requires
additional works (adding a local variable and an argument).
We will finally migrate to use only psref but we can do it
when we have a lockless routing table alternative.
Reference counting for rtentry
------------------------------
rt_refcnt now doesn't count permanent references such as for
rt_timers and rtcaches, instead it is used only for temporal
references when obtaining a rtentry via rtalloc1 and rtrequest{,1}.
We can do so because destroying a rtentry always involves
removing references of rt_timers and rtcaches to the rtentry
and we don't need to track such references. This also makes
it easy to wait for readers to release references on deleting
or updating a rtentry, i.e., we can simply wait until the
reference counter is 0 or 1. (If there are permanent references
the counter can be arbitrary.)
rt_ref increments a reference counter of a rtentry and rt_unref
decrements it. rt_ref is called inside APIs (rtalloc1 and
rtrequest{,1} so users don't need to care about it while
users must call rt_unref to an obtained rtentry after using it.
rtfree is removed and we use rt_unref and rt_free instead.
rt_unref now just decrements the counter of a given rtentry
and rt_free just tries to destroy a given rtentry.
See the next section for destructions of rtentries by rt_free.
Destructions of rtentries
-------------------------
We destroy a rtentry only when we call rtrequst{,1}(RTM_DELETE);
the original implementation can destroy in any rtfree where it's
the last reference. If we use reference counting or psref, it's
easy to understand if the place that a rtentry is destroyed is
fixed.
rt_free waits for references to a given rtentry to be released
before actually destroying the rtentry. rt_free uses a condition
variable (cv_wait) (and psref_target_destroy for psref) to wait.
Unfortunately rtrequst{,1}(RTM_DELETE) can be called in softint
that we cannot use cv_wait. In that case, we have to defer the
destruction to a workqueue.
rtentry#rt_cv, rtentry#rt_psref and global variables
(see rt_free_global) are added to conduct the procedure.
Updates of rtentries
--------------------
One difficulty to use refcnt/psref instead of rwlock for rtentry
is updates of rtentries. We need an additional mechanism to
prevent readers from seeing inconsistency of a rtentry being
updated.
We introduce RTF_UPDATING flag to rtentries that are updating.
While the flag is set to a rtentry, users cannot acquire the
rtentry. By doing so, we avoid users to see inconsistent
rtentries.
There are two options when a user tries to acquire a rtentry
with the RTF_UPDATING flag; if a user runs in softint context
the user fails to acquire a rtentry (NULL is returned).
Otherwise a user waits until the update completes by waiting
on cv.
The procedure of a updater is simpler to destruction of
a rtentry. Wait on cv (and psref) and after all readers left,
proceed with the update.
Global variables (see rt_update_global) are added to conduct
the procedure.
Currently we apply the mechanism to only RTM_CHANGE in
rtsock.c. We would have to apply other codes. See
"Known issues" section.
psref for rtentry
-----------------
When we obtain a rtentry from a rtcache via rtcache_* APIs,
psref is used to reference to the rtentry.
rtcache_ref acquires a reference to a rtentry with psref
and rtcache_unref releases the reference after using it.
rtcache_ref is called inside rtcache_* APIs and users don't
need to take care of it while users must call rtcache_unref
to release the reference.
struct psref and int bound that is needed for psref is
embedded into struct route. By doing so we don't need to
add local variables and additional argument to APIs.
However this adds another constraint to psref other than
reference counting one's; holding a reference of an rtentry
via a rtcache is allowed by just one caller at the same time.
So we must not acquire a rtentry via a rtcache twice and
avoid a recursive use of a rtcache. And also a rtcache must
be arranged to be used by a LWP/softint at the same time
somehow. For IP forwarding case, we have per-CPU rtcaches
used in softint so the constraint is guaranteed. For a h
rtcache of a PCB case, the constraint is guaranteed by the
solock of each PCB. Any other cases (pf, ipf, stf and ipsec)
are currently guaranteed by only the existence of the global
locks (softnet_lock and/or KERNEL_LOCK). If we've found the
cases that we cannot guarantee the constraint, we would need
to introduce other rtcache APIs that use simple reference
counting.
psref of rtcache is created with IPL_SOFTNET and so rtcache
shouldn't used at an IPL higher than IPL_SOFTNET.
Note that rtcache_free is used to invalidate a given rtcache.
We don't need another care by my change; just keep them as
they are.
Performance impact
------------------
When NET_MPSAFE is disabled the performance drop is 3% while
when it's enabled the drop is increased to 11%. The difference
comes from that currently we don't take any global locks and
don't use psref if NET_MPSAFE is disabled.
We can optimize the performance of the case of NET_MPSAFE
on by reducing lookups of rtcache that uses psref;
currently we do two lookups but we should be able to trim
one of two. This is a future work.
Known issues
------------
There are two known issues to be solved; one is that
a caller of rtrequest(RTM_ADD) may change rtentry (see rtinit).
We need to prevent new references during the update. Or
we may be able to remove the code (perhaps, need more
investigations).
The other is rtredirect that updates a rtentry. We need
to apply our update mechanism, however it's not easy because
rtredirect is called in softint and we cannot apply our
mechanism simply. One solution is to defer rtredirect to
a workqueue but it requires some code restructuring.
In the MP-safe world, a rtentry stemming from a rtcache can be freed at any
points. So we need to protect rtentries somehow say by reference couting or
passive references. Regardless of the method, we need to call some release
function of a rtentry after using it.
The change adds a new function rtcache_unref to release a rtentry. At this
point, this function does nothing because for now we don't add a reference
to a rtentry when we get one from a rtcache. We will add something useful
in a further commit.
This change is a part of changes for MP-safe routing table. It is separated
to avoid one big change that makes difficult to debug by bisecting.
nd6_purge depends on that IPv6 addresses are purged. If addresses remain,
pfxlist_onlink_check called from nd6_purge dereferences a dangling pointer
(ia->ia6_ndpr) that is freed before calling pfxlist_onlink_check. Fix it by
removing addresses before calling nd6_purge, which is the original behavior
that was changed by in6.c,v 1.203 and in6_ifattach.c,v 1.99.
Note that it seems the issue occurs because of a hack that forcibly destroys
prefix list entries of a given interface in nd6_purge. We should tackle the
hack in the future.
Fix PR kern/51467
This problem occurs only if NET_MPSAFE on.
ifconfig destroy side:
kernel entry point is ifioctl => if_clone_destroy.
pr_purgeif() acquires softnet_lock, and then ifa_remove() calls
pserialize_perform() holding softnet_lock.
ifconfig side:
kernel entry point is socreate.
pr_attach()(udp_attach_wrapper()) calls sosetlock(). In this call path,
sosetlock() try to acquire softnet_lock.
These can cause dead lock.
Some functions use rt_walktree to scan the routing table and delete
matched routes. However, we shouldn't use rt_walktree to delete
routes because rt_walktree is recursive to the routing table (radix
tree) and isn't friendly to MP-ification. rt_walktree allows a caller
to pass a callback function to delete an matched entry. The callback
function is called from an API of the radix tree (rn_walktree) but
also calls an API of the radix tree to delete an entry.
This change adds a new API of the radix tree, rn_search_matched,
which returns a matched entry that is selected by a callback
function passed by a caller and the caller itself deletes the
entry. By using the API, we can avoid the recursive form.
This change tidies up in6_select* functions, especially
selectroute.
selectroute is annoying because:
- It returns both/either of a rtentry and/or an ifp
- Yes, it may return only an ifp!
- It is valid but selectroute shouldn't handle the case
- Such conditional behavior makes it difficult
to apply locking/psref thingy
- It may return a rtentry even if error
- It may use opt->ip6po_nextroute rtcache implicitly
- The caller can know if it is used
by rtcache_validate(&opt->ip6po_nextroute)
but it's racy in MP-safe world
- Even if it uses opt->ip6po_nextroute, it may
return a rtentry that isn't derived from the rtcache
The change includes:
- Rename selectroute to in6_selectroute
- Let a remaining caller of selectroute, in6_selectif,
use in6_selectroute instead
- Let in6_selectroute return only an rtentry
- If error, it doesn't return an rtentry
- A caller gets an ifp from a returned rtentry
- Allow in6_selectroute to modify a passed rtcache
and a caller can know if opt->ip6po_nextroute is
used via the rtcache
- Let callers (ip6_output and in6_selectif) handle
the case that only an ifp is required
Inspired by OpenBSD
Proposed on tech-kern and tech-net
LGTM by roy@
