Eliminate l->l_ncsw and l->l_nivcsw. From memory think they were added
before we had per-LWP struct rusage; the same is now tracked there.

pserialize(9): Fix buggy assertion inside assertion.

If there has been a context switch, then we can't be in a pserialize
read section -- _and_ whatever we read out of curcpu() is stale, so
we can't assert anything based on it. So just delete the assertion.

Reported-by: syzbot+d7065ab16d7cd9cc09af@syzkaller.appspotmail.com
https://syzkaller.appspot.com/bug?id=0c63db890575d29c6a51fb24b003d3bab1f7f04b

pserialize(9): Fix bug in recent micro-optimization.

curcpu() is always a valid struct cpu_info pointer, but curlwp->l_cpu
is not necessarily -- early at boot on some architectures (like x86,
apparently) it may be null.

Not clear that l->l_cpu is always a cost win over curcpu(): on some
architectures curcpu() is basically a macro for curlwp->l_cpu, while
on others it is a fixed VA. So let's use the one that always works
instead of the one that's sometimes cheaper and sometimes crashier.

pserialize(9): Micro-optimize pserialize_read_exit.

Most of the time we're not cold, so let's convince gcc to make that a
statically predicted-not-taken branch.

pserialize(9): Micro-optimize pserialize_not_in_read_section_p.

Load l_ncsw to test whether we have been preempted, rather than
loading and storing l_nopreempt (via function call) to prevent it.

don't KASSERT(kpreempt_disabled()) while cold
pserialize_read_*() can be called *very* early in kernel startup these days

pserialize(9): Lift rule that pserialize_perform be serialized.

There may have been a technical reason for this back when we were
following the expired passive serialization patent to the letter, but
no more -- and this is a real burden for some applications.

Switch psz_ev_excl to static evcnt.

Restore psz_lock just for the event count.

Cost of mutex_enter/exit is negligible compared to the xcall we just
did, so this is not going to meaningfully affect performance.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

pserialize(9): Fix buggy assertion inside assertion.

If there has been a context switch, then we can't be in a pserialize
read section -- _and_ whatever we read out of curcpu() is stale, so
we can't assert anything based on it. So just delete the assertion.

Reported-by: syzbot+d7065ab16d7cd9cc09af@syzkaller.appspotmail.com
https://syzkaller.appspot.com/bug?id=0c63db890575d29c6a51fb24b003d3bab1f7f04b

pserialize(9): Fix bug in recent micro-optimization.

curcpu() is always a valid struct cpu_info pointer, but curlwp->l_cpu
is not necessarily -- early at boot on some architectures (like x86,
apparently) it may be null.

Not clear that l->l_cpu is always a cost win over curcpu(): on some
architectures curcpu() is basically a macro for curlwp->l_cpu, while
on others it is a fixed VA. So let's use the one that always works
instead of the one that's sometimes cheaper and sometimes crashier.

pserialize(9): Micro-optimize pserialize_read_exit.

Most of the time we're not cold, so let's convince gcc to make that a
statically predicted-not-taken branch.

pserialize(9): Micro-optimize pserialize_not_in_read_section_p.

Load l_ncsw to test whether we have been preempted, rather than
loading and storing l_nopreempt (via function call) to prevent it.

don't KASSERT(kpreempt_disabled()) while cold
pserialize_read_*() can be called *very* early in kernel startup these days

pserialize(9): Lift rule that pserialize_perform be serialized.

There may have been a technical reason for this back when we were
following the expired passive serialization patent to the letter, but
no more -- and this is a real burden for some applications.

Switch psz_ev_excl to static evcnt.

Restore psz_lock just for the event count.

Cost of mutex_enter/exit is negligible compared to the xcall we just
did, so this is not going to meaningfully affect performance.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

don't KASSERT(kpreempt_disabled()) while cold
pserialize_read_*() can be called *very* early in kernel startup these days

pserialize(9): Lift rule that pserialize_perform be serialized.

There may have been a technical reason for this back when we were
following the expired passive serialization patent to the letter, but
no more -- and this is a real burden for some applications.

Switch psz_ev_excl to static evcnt.

Restore psz_lock just for the event count.

Cost of mutex_enter/exit is negligible compared to the xcall we just
did, so this is not going to meaningfully affect performance.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

pserialize(9): Lift rule that pserialize_perform be serialized.

There may have been a technical reason for this back when we were
following the expired passive serialization patent to the letter, but
no more -- and this is a real burden for some applications.

Switch psz_ev_excl to static evcnt.

Restore psz_lock just for the event count.

Cost of mutex_enter/exit is negligible compared to the xcall we just
did, so this is not going to meaningfully affect performance.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

Switch psz_ev_excl to static evcnt.

Restore psz_lock just for the event count.

Cost of mutex_enter/exit is negligible compared to the xcall we just
did, so this is not going to meaningfully affect performance.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

Stopgap hack to unbreak the build: #ifdef __HAVE_ATOMIC64_LOADSTORE
the event counter update. From rmind@

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.

Rip out pserialize(9) logic now that the RCU patent has expired.

pserialize_perform() is now basically just xc_barrier(XC_HIGHPRI).
No more tentacles throughout the scheduler. Simplify the psz read
count for diagnostic assertions by putting it unconditionally into
cpu_info.

From rmind@, tidied up by me.

xc_barrier - convenience function to xc_broadcast() a nop.

Make the intent more clear and also avoid a bunch of (xcfunc_t)nullop
casts that gcc 8 -Wcast-function-type is not happy about.

Change the place to check if a context switch doesn't happen within a pserialize read section

The previous place (pserialize_switchpoint) was not a good place because at that
point a suspect thread is already switched so that a backtrace gotten on
a KASSERT failure doesn't point out where a context switch happens.

branches: 1.11.2;
Function pserialize_perform() usually succeeds after two cross calls
so defer kpause() to iterations three and above.

Speeds up VOP_REVOKE() on /proc/XXX/status by a factor of ~12.

Ok: core@

branches: 1.10.2;
Prevent panic or hangup in softint_disestablish(), pserialize_perform() or
psref_target_destroy() while mp_online == false.

See http://mail-index.netbsd.org/tech-kern/2017/12/25/msg022829.html

Implement debugging feature for pserialize(9)

The debugging feature detects violations of pserialize constraints.
It causes a panic:
- if a context switch happens in a read section, or
- if a sleepable function is called in a read section.

The feature is enabled only if LOCKDEBUG is on.

Discussed on tech-kern@

branches: 1.8.10;
Typo in comment.

branches: 1.7.14;
- pserialize_switchpoint: check for passing twice, not more than needed.
- pserialize_perform: avoid a possible race with softint handler.
Reported by hannken@.

- softint_dispatch: perform pserialize(9) switchpoint when softintr processing
finishes (without blocking). Problem reported by hannken@, thanks!
- pserialize_read_enter: use splsoftserial(), not splsoftclock().
- pserialize_perform: add xcall(9) barrier as interrupts may be coalesced.

branches: 1.5.2; 1.5.6; 1.5.8;
- Add mi_cpu_init() and initialise cpu_lock and kcpuset_attached/running there.
- Add kcpuset_running which gets set in idle_loop().
- Use kcpuset_running in pserialize_perform().

branches: 1.4.2; 1.4.6;
- Add an argument to kcpuset_create() for zeroing.
- Add kcpuset_atomic_set(), kcpuset_atomic_clear() and kcpuset_merge().

Add kcpuset(9) - a reworked dynamic CPU set implementation for kernel.
Suitable for use during the early boot. MD and other implementations
should be replaced with this interface.

Discussed on: tech-kern@

Need to include <sys/evcnt.h> explicitly to build this on at least amiga.

Add an implementation of passive serialization as described in expired
US patent 4809168. This is a reader / writer synchronization mechanism,
designed for lock-less read operations.