Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC 261517,261520:
Convert the license on files where I am the sole copyright holder to
2 clause BSD licenses.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Consistently use the same value to indicate exclusively-held and
shared-held locks for all the primitives in lc_lock/lc_unlock routines.
This fixes the problems introduced in r255747, which indeed introduced an
inversion in the logic.

Reported by: many
Tested by: bdrewery, pho, lme, Adam McDougall, O. Hartmann
Approved by: re (glebius)

Fix lc_lock/lc_unlock() support for rmlocks held in shared mode. With
current lock classes KPI it was really difficult because there was no
way to pass an rmtracker object to the lock/unlock routines. In order
to accomplish the task, modify the aforementioned functions so that
they can return (or pass as argument) an uinptr_t, which is in the rm
case used to hold a pointer to struct rm_priotracker for current
thread. As an added bonus, this fixes rm_sleep() in the rm shared
case, which right now can communicate priotracker structure between
lc_unlock()/lc_lock().

Suggested by: jhb
Reviewed by: jhb
Approved by: re (delphij)

A few mostly cosmetic nits to aid in debugging:
- Call lock_init() first before setting any lock_object fields in
lock init routines. This way if the machine panics due to a duplicate
init the lock's original state is preserved.
- Somewhat similarly, don't decrement td_locks and td_slocks until after
an unlock operation has completed successfully.

- Handle the recursed/not recursed flags with RA_RLOCKED in rw_assert().
- Tweak a panic message.

Fixup r240424: On entering KDB backends, the hijacked thread to run
interrupt context can still be idlethread. At that point, without the
panic condition, it can still happen that idlethread then will try to
acquire some locks to carry on some operations.

Skip the idlethread check on block/sleep lock operations when KDB is
active.

Reported by: jh
Tested by: jh
MFC after: 1 week

Merge r242395,242483 from mutex implementation:
give rwlock(9) the ability to crunch different type of structures, with
the only constraint that they have a lock cookie named rw_lock.
This name, then, becames reserved from the struct that wants to use
the rwlock(9) KPI and other locking primitives cannot reuse it for
their members.

Namely such structs are the current struct rwlock and the new struct
rwlock_padalign. The new structure will define an object which has the
same layout of a struct rwlock but will be allocated in areas aligned
to the cache line size and will be as big as a cache line.

For further details check comments on above mentioned revisions.

Reviewed by: jimharris, jeff

Remove all the checks on curthread != NULL with the exception of some MD
trap checks (eg. printtrap()).

Generally this check is not needed anymore, as there is not a legitimate
case where curthread != NULL, after pcpu 0 area has been properly
initialized.

Reviewed by: bde, jhb
MFC after: 1 week

Improve check coverage about idle threads.

Idle threads are not allowed to acquire any lock but spinlocks.
Deny any attempt to do so by panicing at the locking operation
when INVARIANTS is on. Then, remove the check on blocking on a
turnstile.
The check in sleepqueues is left because they are not allowed to use
tsleep() either which could happen still.

Reviewed by: bde, jhb, kib
MFC after: 1 week

Add software PMC support.

New kernel events can be added at various location for sampling or counting.
This will for example allow easy system profiling whatever the processor is
with known tools like pmcstat(8).

Simultaneous usage of software PMC and hardware PMC is possible, for example
looking at the lock acquire failure, page fault while sampling on
instructions.

Sponsored by: NETASQ
MFC after: 1 month

panic: add a switch and infrastructure for stopping other CPUs in SMP case

Historical behavior of letting other CPUs merily go on is a default for
time being. The new behavior can be switched on via
kern.stop_scheduler_on_panic tunable and sysctl.

Stopping of the CPUs has (at least) the following benefits:
- more of the system state at panic time is preserved intact
- threads and interrupts do not interfere with dumping of the system
state

Only one thread runs uninterrupted after panic if stop_scheduler_on_panic
is set. That thread might call code that is also used in normal context
and that code might use locks to prevent concurrent execution of certain
parts. Those locks might be held by the stopped threads and would never
be released. To work around this issue, it was decided that instead of
explicit checks for panic context, we would rather put those checks
inside the locking primitives.

This change has substantial portions written and re-written by attilio
and kib at various times. Other changes are heavily based on the ideas
and patches submitted by jhb and mdf. bde has provided many insights
into the details and history of the current code.

The new behavior may cause problems for systems that use a USB keyboard
for interfacing with system console. This is because of some unusual
locking patterns in the ukbd code which have to be used because on one
hand ukbd is below syscons, but on the other hand it has to interface
with other usb code that uses regular mutexes/Giant for its concurrency
protection. Dumping to USB-connected disks may also be affected.

PR: amd64/139614 (at least)
In cooperation with: attilio, jhb, kib, mdf
Discussed with: arch@, bde
Tested by: Eugene Grosbein <eugen@grosbein.net>,
gnn,
Steven Hartland <killing@multiplay.co.uk>,
glebius,
Andrew Boyer <aboyer@averesystems.com>
(various versions of the patch)
MFC after: 3 months (or never)

Constify arguments for locking KPIs where possible.

This enables locking consumers to pass their own structures around as const and
be able to assert locks embedded into those structures.

Reviewed by: ed, kib, jhb

Mark all SYSCTL_NODEs static that have no corresponding SYSCTL_DECLs.

The SYSCTL_NODE macro defines a list that stores all child-elements of
that node. If there's no SYSCTL_DECL macro anywhere else, there's no
reason why it shouldn't be static.

Print the pointer to the lock with the panic message. The previous
panic: rw lock not unlocked
was not really helpful for debugging. Now one can at least call
show lock <ptr>
form ddb to learn more about the lock.

MFC after: 3 days

When releasing a read/shared lock we need to use a write memory barrier
in order to avoid, on architectures which doesn't have strong ordered
writes, CPU instructions reordering.

Diagnosed by: fabio
Reviewed by: jhb
Tested by: Giovanni Trematerra
<giovanni dot trematerra at gmail dot com>

* Change the scope of the ASSERT_ATOMIC_LOAD() from a generic check to
a pointer-fetching specific operation check. Consequently, rename the
operation ASSERT_ATOMIC_LOAD_PTR().
* Fix the implementation of ASSERT_ATOMIC_LOAD_PTR() by checking
directly alignment on the word boundry, for all the given specific
architectures. That's a bit too strict for some common case, but it
assures safety.
* Add a comment explaining the scope of the macro
* Add a new stub in the lockmgr specific implementation

Tested by: marcel (initial version), marius
Reviewed by: rwatson, jhb (comment specific review)
Approved by: re (kib)

Add a new macro to test that a variable could be loaded atomically.
Check that the given variable is at most uintptr_t in size and that
it is aligned.

Note: ASSERT_ATOMIC_LOAD() uses ALIGN() to check for adequate
alignment -- however, the function of ALIGN() is to guarantee
alignment, and therefore may lead to stronger alignment
enforcement than necessary for types that are smaller than
sizeof(uintptr_t).

Add checks to mtx, rw and sx locks init functions to detect possible
breakage. This was used during debugging of the problem fixed with
r196118 where a pointer was on an un-aligned address in the dpcpu area.

In collaboration with: rwatson
Reviewed by: rwatson
Approved by: re (kib)

Handle lock recursion differenty by always checking against LO_RECURSABLE
instead the lock own flag itself.

Tested by: pho

Remove extra cpu_spinwait() invocations. This should really only be used
in tight spin loops, not in these edge cases where we restart a much
larger loop only a few times.

Reviewed by: attilio

Tweak a few comments on adaptive spinning.

Add the OpenSolaris dtrace lockstat provider. The lockstat provider
adds probes for mutexes, reader/writer and shared/exclusive locks to
gather contention statistics and other locking information for
dtrace scripts, the lockstat(1M) command and other potential
consumers.

Reviewed by: attilio jhb jb
Approved by: gnn (mentor)

- Wrap lock profiling state variables in #ifdef LOCK_PROFILING blocks.

Remove even more unneeded variable assignments.

kern_time.c:
- Unused variable `p'.

kern_thr.c:
- Variable `error' is always caught immediately, so no reason to
initialize it. There is no way that error != 0 at the end of
create_thread().

kern_sig.c:
- Unused variable `code'.

kern_synch.c:
- `rval' is always assigned in all different cases.

kern_rwlock.c:
- `v' is always overwritten with RW_UNLOCKED further on.

kern_malloc.c:
- `size' is always initialized with the proper value before being used.

kern_exit.c:
- `error' is always caught and returned immediately. abort2() never
returns a non-zero value.

kern_exec.c:
- `len' is always assigned inside the if-statement right below it.

tty_info.c:
- `td' is always overwritten by FOREACH_THREAD_IN_PROC().

Found by: LLVM's scan-build

add RW_SYSINIT_FLAGS macro and rw_sysinit_flags initialization function

Teach WITNESS about the interlocks used with lockmgr. This removes a bunch
of spurious witness warnings since lockmgr grew witness support. Before
this, every time you passed an interlock to a lockmgr lock WITNESS treated
it as a LOR.

Reviewed by: attilio

Various whitespace fixes.

Improve a comment which, in the actual CVS stock, doesn't completely
explain the logic of the code chunk.

- Add sysctls at debug.rwlock to control the behavior of the speculative
spinning when readers hold a lock. This spinning is speculative because,
unlike the write case, we can not test whether the owners are running.
- Add speculative read spinning for readers who are blocked by pending
writers while a read lock is still held. This allows the thread to
spin until the write lock succeeds after which it may spin until the
writer has released the lock. This prevents excessive context switches
when readers and writers both hold the lock for brief periods.

Sponsored by: Nokia

Add rw_try_rlock() and rw_try_wlock() to rwlocks.
These functions try the specified operation (rlocking and wlocking) and
true is returned if the operation completes, false otherwise.

The KPI is enriched by this commit, so __FreeBSD_version bumping and
manpage updating will happen soon.

Requested by: jeff, kris

- In rw_wunlock_hard prefer to wakeup writers if there are both readers
and writers available. Doing otherwise can cause deadlocks as no
read locks can proceed while there are write waiters.

Sponsored by: Nokia

Adaptive spinning in write path with readers and writer starvation avoidance.
- Move recursion checking into rwlock inlines to free a bit for use with
adaptive spinners.
- Clear the RW_LOCK_WRITE_SPINNERS flag whenever the lock state changes
causing write spinners to restart their loop.
- Write spinners are limited by a count while readers hold the lock as
there is no way to know for certain whether readers are running still.
- In the read path block if there are write waiters or spinners to avoid
starving writers. Use a new per-thread count, td_rw_rlocks, to skip
starvation avoidance if it might cause a deadlock.
- Remove or change invalid assertions in turnstiles.

Reviewed by: attilio (developed parts of the patch as well)
Sponsored by: Nokia

Remove a conditional that is always true.

MFC after: 2 weeks

- Re-implement lock profiling in such a way that it no longer breaks
the ABI when enabled. There is no longer an embedded lock_profile_object
in each lock. Instead a list of lock_profile_objects is kept per-thread
for each lock it may own. The cnt_hold statistic is now always 0 to
facilitate this.
- Support shared locking by tracking individual lock instances and
statistics in the per-thread per-instance lock_profile_object.
- Make the lock profiling hash table a per-cpu singly linked list with a
per-cpu static lock_prof allocator. This removes the need for an array
of spinlocks and reduces cache contention between cores.
- Use a seperate hash for spinlocks and other locks so that only a
critical_enter() is required and not a spinlock_enter() to modify the
per-cpu tables.
- Count time spent spinning in the lock statistics.
- Remove the LOCK_PROFILE_SHARED option as it is always supported now.
- Specifically drop and release the scheduler locks in both schedulers
since we track owners now.

In collaboration with: Kip Macy
Sponsored by: Nokia

Simplify the adaptive spinning algorithm in rwlock and mutex:
currently, before to spin the turnstile spinlock is acquired and the
waiters flag is set.
This is not strictly necessary, so just spin before to acquire the
spinlock and to set the flags.
This will simplify a lot other functions too, as now we have the waiters
flag set only if there are actually waiters.
This should make wakeup/sleeping couplet faster under intensive mutex
workload.
This also fixes a bug in rw_try_upgrade() in the adaptive case, where
turnstile_lookup() will recurse on the ts_lock lock that will never be
really released [1].

[1] Reported by: jeff with Nokia help
Tested by: pho, kris (earlier, bugged version of rwlock part)
Discussed with: jhb [2], jeff
MFC after: 1 week

[2] John had a similar patch about 6.x and/or 7.x about mutexes probabilly

Expand lock class with the "virtual" function lc_assert which will offer
an unified way for all the lock primitives to express lock assertions.
Currenty, lockmgrs and rmlocks don't have assertions, so just panic in
that case.
This will be a base for more callout improvements.

Ok'ed by: jhb, jeff

Remove a bogus KASSERT which will prevent rwlock to be acquired
recursively in exclusive mode with debugging kernels.

Submitted by: kmacy
Approved by: jeff

generally we are interested in what thread did something as
opposed to what process. Since threads by default have teh name of the
process unless over-written with more useful information, just print the
thread name instead.

Fix some problems with lock profiling in rw locks:
- Adjust lock_profiling stubs semantic in the hard functions in order to be
more accurate and trustable
- As for sx locks, disable shared paths for lock_profiling. Actually,
lock_profiling has a subtle race which makes results caming from shared
paths not completely trustable. A macro stub (LOCK_PROFILING_SHARED) can
be actually used for re-enabling this paths, but is currently intended
for developing use only.
- style(9) fixes

Approved by: jeff, kmacy, jhb[1]
Approved by: re

[1] Had initial reservations not shared by others, conceded
in the end.

Introduce a new rwlocks initialization function: rw_init_flags.
This is very similar to sx_init_flags: it initializes the rwlock using
special flags passed as third argument (RW_DUPOK, RW_NOPROFILE,
RW_NOWITNESS, RW_QUIET, RW_RECURSE).
Among these, the most important new feature is probabilly that rwlocks
can be acquired recursively now (for both shared and exclusive paths).

Because of the recursion counter, the ABI is changed.

Tested by: Timothy Redaelli <drizzt@gufi.org>
Reviewed by: jhb
Approved by: jeff (mentor)
Approved by: re

Commit 3/14 of sched_lock decomposition.
- Add a per-turnstile spinlock to solve potential priority propagation
deadlocks that are possible with thread_lock().
- The turnstile lock order is defined as the exact opposite of the
lock order used with the sleep locks they represent. This allows us
to walk in reverse order in priority_propagate and this is the only
place we wish to multiply acquire turnstile locks.
- Use the turnstile_chain lock to protect assigning mutexes to turnstiles.
- Change the turnstile interface to pass back turnstile pointers to the
consumers. This allows us to reduce some locking and makes it easier
to cancel turnstile assignment while the turnstile chain lock is held.

Tested by: kris, current@
Tested on: i386, amd64, ULE, 4BSD, libthr, libkse, PREEMPTION, etc.
Discussed with: kris, attilio, kmacy, jhb, julian, bde (small parts each)

Move lock_profile_object_{init,destroy}() into lock_{init,destroy}().

Add destroyed cookie values for sx locks and rwlocks as well as extra
KASSERTs so that any lock operations on a destroyed lock will panic or
hang.

- Drop memory barriers in rw_try_upgrade(). We don't need an 'acq' memory
barrier here as the earlier rw_rlock() already contained one.
- Comment fix.

- Simplify the #ifdef's for adaptive mutexes and rwlocks by conditionally
defining a macro earlier in the file.
- Add NO_ADAPTIVE_RWLOCKS option to disable adaptive spinning for rwlocks.

Rename the 'mtx_object', 'rw_object', and 'sx_object' members of mutexes,
rwlocks, and sx locks to 'lock_object'.

Print readers count as unsigned in ddb 'show lock'.

Submitted by: attilio

Fix a typo.

Add two new function pointers 'lc_lock' and 'lc_unlock' to lock classes.
These functions are intended to be used to drop a lock and then reacquire
it when doing an sleep such as msleep(9). Both functions accept a
'struct lock_object *' as their first parameter. The 'lc_unlock' function
returns an integer that is then passed as the second paramter to the
subsequent 'lc_lock' function. This can be used to communicate state.
For example, sx locks and rwlocks use this to indicate if the lock was
share/read locked vs exclusive/write locked.

Currently, spin mutexes and lockmgr locks do not provide working lc_lock
and lc_unlock functions.

Use C99-style struct member initialization for lock classes.

Fix some nits in lock profiling for rwlocks:
- Properly note when a read lock is released.
- Always note when we contest on a read lock.
- Only note success of obtaining read locks for the first reader to match
the behavior of sx(9).

Reviewed by: kmacy

Further improvements to LOCK_PROFILING:
- Fix missing initialization in kern_rwlock.c causing bogus times to be collected
- Move updates to the lock hash to after the lock is released for spin mutexes,
sleep mutexes, and sx locks
- Add new kernel build option LOCK_PROFILE_FAST - only update lock profiling
statistics when an acquisition is contended. This reduces the overhead of
LOCK_PROFILING to increasing system time by 20%-25% which on
"make -j8 kernel-toolchain" on a dual woodcrest is unmeasurable in terms
of wall-clock time. Contrast this to enabling lock profiling without
LOCK_PROFILE_FAST and I see a 5x-6x slowdown in wall-clock time.

Add rw_wowned() interface to rwlock(9), allowing a kernel thread to
determine if it holds an exclusive rwlock reference or not. This is
non-ideal, but recursion scenarios in the network stack currently
require it.

Approved by: jhb

general LOCK_PROFILING cleanup

- only collect timestamps when a lock is contested - this reduces the overhead
of collecting profiles from 20x to 5x

- remove unused function from subr_lock.c

- generalize cnt_hold and cnt_lock statistics to be kept for all locks

- NOTE: rwlock profiling generates invalid statistics (and most likely always has)
someone familiar with that should review

track lock class name in a way that doesn't break WITNESS

MUTEX_PROFILING has been generalized to LOCK_PROFILING. We now profile
wait (time waited to acquire) and hold times for *all* kernel locks. If
the architecture has a system synchronized TSC, the profiling code will
use that - thereby minimizing profiling overhead. Large chunks of profiling
code have been moved out of line, the overhead measured on the T1 for when
it is compiled in but not enabled is < 1%.

Approved by: scottl (standing in for mentor rwatson)
Reviewed by: des and jhb

Adjust td_locks for non-spin mutexes, rwlocks, and sx locks so that it is
a count of all non-spin locks, not just lockmgr locks. This can give us a
much cheaper way to see if we have any locks held (such as when returning
to userland via userret()) without requiring WITNESS.

MFC after: 1 week

Implement rw_try_upgrade() and rw_downgrade(). rw_try_upgrade() makes a
single attempt at upgrading a read lock to a write lock, and rw_downgrade()
converts curthread's write lock into a read lock.

'owner' is not used without SMP. Fix kernel build for such kernel
configurations.

Approved by: jhb

Adaptively spin before blocking on the turnstile if an rwlock is write
locked. In general the adaptive spinning is similar to the same code
for mutexes with some extra trickiness in rw_wunlock_hard(). Specifically,
even though both wait bits might be set and we might have a turnstile with
at least one waiting thread, there might not be any threads blocked on the
queue we are not waking up (they might all be spinning), and we should
only preserve the waiting flag for the queue we aren't waking up if there
are in fact threads blocked on that queue. Secondly, there might not be
any threads blocked on the queue we have chosen to waken threads from
(there might only be threads blocked on the other queue and the threads
for this queue are all spinning) in which case we disown the turnstile
instead of doing a braodcast and unpend.

- Add a rw_wowner() macro that just returns the owner of a write lock and
use it in places that only care about the write owner instead of
rw_owner() as a baby step towards limited read-lock owner.
- Tidy the code that sets the WAITER flag bits to not duplicate a test
around the atomic operation and the KTR trace in both of the lock
functions.

Fix another compile problem. If I find any more, this file is going in the
Attic until it is properly fixed.

Regroup order of operations to better reflect what was probably intended.

Submitted by: Peter Jeremy

Take a stab at making this compile when WITNESS is not defined. gcc can't
figure out the order of operations at line 519, and neither can I, but this
is my best guess. Also correct a number of typos and syntax errors.

Unbreak on archs where %d doesn't print uintptr_t arithmetic.

Add a basic reader/writer lock implementation to the kernel. This
implementation is by no means perfect as far as some of the algorithms
that it uses and the fact that it is missing some functionality (try
locks and upgrades/downgrades are not there yet), however it does seem
to work in my local testing. There is more detail in the comments in the
code, but the short version follows.

A reader/writer lock is very much like a regular mutex: it cannot be held
across a voluntary sleep; it can be acquired in an interrupt thread; if
the lock is held by a writer then the priority of any threads that block
on the lock will be lent to the owner; the simple case lock operations all
are done in a single atomic op. It also shares some similiarities
with sx locks: it supports reader/writer semantics (multiple readers,
but single writers); readers are allowed to recurse, but writers are not.

We can extend this implementation further by either improving algorithms
or adding new functionality, but this should at least give us a base to
work with now.

Reviewed by: arch (in theory)
Tested on: i386 (4 cpu box with a kernel module that used 4 threads
that randomly chose between read locks and write locks
that ran w/o panicing for over a day solid. It usually
panic'd within a few seconds when there were bugs during
testing. :) The kernel module source is available on
request.)