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/design/trunk/src/ch_synchronization.xml
41,19 → 41,18
remains unavailable until this operation called on the respective
spinlock returns zero. HelenOS builds two functions on top of the
test-and-set operation. The first function is the unconditional attempt
to acquire the spinlock and is called
<emphasis>spinlock_lock</emphasis>. It simply loops until the
test-and-set returns a zero value. The other function,
<emphasis>spinlock_trylock</emphasis>, is the conditional lock operation
and calls the test-and-set only once to find out whether it managed to
acquire the spinlock or not. The conditional operation is useful in
situations in which an algorithm cannot acquire more spinlocks in the
proper order and a deadlock cannot be avoided. In such a case, the
algorithm would detect the danger and instead of possibly deadlocking
the system it would simply release some spinlocks it already holds and
retry the whole operation with the hope that it will succeed next time.
The unlock function, <emphasis>spinlock_unlock</emphasis>, is quite easy
- it merely clears the spinlock variable.</para>
to acquire the spinlock and is called <code>spinlock_lock</code>. It
simply loops until the test-and-set returns a zero value. The other
function, <code>spinlock_trylock</code>, is the conditional lock
operation and calls the test-and-set only once to find out whether it
managed to acquire the spinlock or not. The conditional operation is
useful in situations in which an algorithm cannot acquire more spinlocks
in the proper order and a deadlock cannot be avoided. In such a case,
the algorithm would detect the danger and instead of possibly
deadlocking the system it would simply release some spinlocks it already
holds and retry the whole operation with the hope that it will succeed
next time. The unlock function, <code>spinlock_unlock</code>, is quite
easy - it merely clears the spinlock variable.</para>
 
<para>Nevertheless, there is a special issue related to hardware
optimizations that modern processors implement. Particularly problematic
66,20 → 65,19
respective spinlock is not implicit on some processor architectures. As
a result, the processor needs to be explicitly told about each
occurrence of such a dependency. Therefore, HelenOS adds
architecture-specific hooks to all <emphasis>spinlock_lock</emphasis>,
<emphasis>spinlock_trylock</emphasis> and
<emphasis>spinlock_unlock</emphasis> functions to prevent the
instructions inside the critical section from permeating out. On some
architectures, these hooks can be void because the dependencies are
implicitly there because of the special properties of locking and
unlocking instructions. However, other architectures need to instrument
these hooks with different memory barriers, depending on what operations
could permeate out.</para>
architecture-specific hooks to all <code>spinlock_lock</code>,
<code>spinlock_trylock</code> and <code>spinlock_unlock</code> functions
to prevent the instructions inside the critical section from permeating
out. On some architectures, these hooks can be void because the
dependencies are implicitly there because of the special properties of
locking and unlocking instructions. However, other architectures need to
instrument these hooks with different memory barriers, depending on what
operations could permeate out.</para>
 
<para>Spinlocks have one significant drawback: when held for longer time
periods, they harm both parallelism and concurrency. The processor
executing <emphasis>spinlock_lock</emphasis> does not do any fruitful
work and is effectively halted until it can grab the lock and proceed.
executing <code>spinlock_lock</code> does not do any fruitful work and
is effectively halted until it can grab the lock and proceed.
Similarily, other execution flows cannot execute on the processor that
holds the spinlock because the kernel disables preemption on that
processor when a spinlock is held. The reason behind disabling
114,25 → 112,24
queue are protected by a spinlock.</para>
 
<para>The thread that wants to wait for a wait queue event uses the
<emphasis>waitq_sleep_timeout</emphasis> function. The algorithm then
checks the wait queue's counter of missed wakeups and if there are any
missed wakeups, the call returns immediately. The call also returns
immediately if only a conditional wait was requested. Otherwise the
thread is enqueued in the wait queue's list of sleeping threads and its
state is changed to <emphasis>Sleeping</emphasis>. It then sleeps until
one of the following events happens:</para>
<code>waitq_sleep_timeout</code> function. The algorithm then checks the
wait queue's counter of missed wakeups and if there are any missed
wakeups, the call returns immediately. The call also returns immediately
if only a conditional wait was requested. Otherwise the thread is
enqueued in the wait queue's list of sleeping threads and its state is
changed to <constant>Sleeping</constant>. It then sleeps until one of
the following events happens:</para>
 
<orderedlist>
<listitem>
<para>another thread calls <emphasis>waitq_wakeup</emphasis> and the
thread is the first thread in the wait queue's list of sleeping
<para>another thread calls <code>waitq_wakeup</code> and the thread
is the first thread in the wait queue's list of sleeping
threads;</para>
</listitem>
 
<listitem>
<para>another thread calls
<emphasis>waitq_interrupt_sleep</emphasis> on the sleeping
thread;</para>
<para>another thread calls <code>waitq_interrupt_sleep</code> on the
sleeping thread;</para>
</listitem>
 
<listitem>
144,28 → 141,27
 
<para>All five possibilities (immediate return on success, immediate
return on failure, wakeup after sleep, interruption and timeout) are
distinguishable by the return value of
<emphasis>waitq_sleep_timeout</emphasis>. Being able to interrupt a
sleeping thread is essential for externally initiated thread
termination. The ability to wait only for a certain amount of time is
used, for instance, to passively delay thread execution by several
microseconds or even seconds in <emphasis>thread_sleep</emphasis>
function. Due to the fact that all other passive kernel synchronization
primitives are based on wait queues, they also have the option of being
interrutped and, more importantly, can timeout. All of them also
implement the conditional operation. Furthemore, this very fundamental
interface reaches up to the implementation of futexes - userspace
synchronization primitive, which makes it possible for a userspace
thread to request a synchronization operation with a timeout or a
conditional operation.</para>
distinguishable by the return value of <code>waitq_sleep_timeout</code>.
Being able to interrupt a sleeping thread is essential for externally
initiated thread termination. The ability to wait only for a certain
amount of time is used, for instance, to passively delay thread
execution by several microseconds or even seconds in
<code>thread_sleep</code> function. Due to the fact that all other
passive kernel synchronization primitives are based on wait queues, they
also have the option of being interrutped and, more importantly, can
timeout. All of them also implement the conditional operation.
Furthemore, this very fundamental interface reaches up to the
implementation of futexes - userspace synchronization primitive, which
makes it possible for a userspace thread to request a synchronization
operation with a timeout or a conditional operation.</para>
 
<para>From the description above, it should be apparent, that when a
sleeping thread is woken by <emphasis>waitq_wakeup</emphasis> or when
<emphasis>waitq_sleep_timeout</emphasis> succeeds immediately, the
thread can be sure that the event has occurred. The thread need not and
should not verify this fact. This approach is called direct hand-off and
is characteristic for all passive HelenOS synchronization primitives,
with the exception as described below.</para>
sleeping thread is woken by <code>waitq_wakeup</code> or when
<code>waitq_sleep_timeout</code> succeeds immediately, the thread can be
sure that the event has occurred. The thread need not and should not
verify this fact. This approach is called direct hand-off and is
characteristic for all passive HelenOS synchronization primitives, with
the exception as described below.</para>
</section>
 
<section>
173,24 → 169,23
 
<para>The interesting point about wait queues is that the number of
missed wakeups is equal to the number of threads that will not block in
<emphasis>watiq_sleep_timeout</emphasis> and would immediately succeed
instead. On the other hand, semaphores are synchronization primitives
that will let predefined amount of threads into their critical section
and block any other threads above this count. However, these two cases
are exactly the same. Semaphores in HelenOS are therefore implemented as
wait queues with a single semantic change: their wait queue is
initialized to have so many missed wakeups as is the number of threads
that the semphore intends to let into its critical section
simultaneously.</para>
<code>watiq_sleep_timeout</code> and would immediately succeed instead.
On the other hand, semaphores are synchronization primitives that will
let predefined amount of threads into their critical section and block
any other threads above this count. However, these two cases are exactly
the same. Semaphores in HelenOS are therefore implemented as wait queues
with a single semantic change: their wait queue is initialized to have
so many missed wakeups as is the number of threads that the semphore
intends to let into its critical section simultaneously.</para>
 
<para>In the semaphore language, the wait queue operation
<emphasis>waitq_sleep_timeout</emphasis> corresponds to
<emphasis><emphasis>semaphore</emphasis> down</emphasis> operation,
represented by the function <emphasis>semaphore_down_timeout</emphasis>
and by way of similitude the wait queue operation waitq_wakeup
corresponds to semaphore <emphasis>up</emphasis> operation, represented
by the function <emphasis>sempafore_up</emphasis>. The conditional down
operation is called <emphasis>semaphore_trydown</emphasis>.</para>
<code>waitq_sleep_timeout</code> corresponds to semaphore
<code>down</code> operation, represented by the function
<code>semaphore_down_timeout</code> and by way of similitude the wait
queue operation waitq_wakeup corresponds to semaphore <code>up</code>
operation, represented by the function <code>sempafore_up</code>. The
conditional down operation is called
<code>semaphore_trydown</code>.</para>
</section>
 
<section>
203,9 → 198,9
view, they can be viewed as spinlocks without busy waiting. Their
semaphore heritage provides good basics for both conditional operation
and operation with timeout. The locking operation is called
<emphasis>mutex_lock</emphasis>, the conditional locking operation is
called <emphasis>mutex_trylock</emphasis> and the unlocking operation is
called <emphasis>mutex_unlock</emphasis>.</para>
<code>mutex_lock</code>, the conditional locking operation is called
<code>mutex_trylock</code> and the unlocking operation is called
<code>mutex_unlock</code>.</para>
</section>
 
<section>
256,10 → 251,10
<para>The implementation of rwlocks as it has been already put, makes
use of one single wait queue for both readers and writers, thus avoiding
any possibility of starvation. In fact, rwlocks use a mutex rather than
a bare wait queue. This mutex is called <emphasis>exclusive</emphasis>
and is used to synchronize writers. The writer's lock operation,
<emphasis>rwlock_write_lock_timeout</emphasis>, simply tries to acquire
the exclusive mutex. If it succeeds, the writer is granted the rwlock.
a bare wait queue. This mutex is called <code>exclusive</code> and is
used to synchronize writers. The writer's lock operation,
<code>rwlock_write_lock_timeout</code>, simply tries to acquire the
exclusive mutex. If it succeeds, the writer is granted the rwlock.
However, if the operation fails (e.g. times out), the writer must check
for potential readers at the head of the list of sleeping threads
associated with the mutex's wait queue and then proceed according to the
267,8 → 262,8
 
<para>The exclusive mutex plays an important role in reader
synchronization as well. However, a reader doing the reader's lock
operation, <emphasis>rwlock_read_lock_timeout</emphasis>, may bypass
this mutex when it detects that:</para>
operation, <code>rwlock_read_lock_timeout</code>, may bypass this mutex
when it detects that:</para>
 
<orderedlist>
<listitem>
314,31 → 309,28
<function>condvar_signal</function>(<varname>cv</varname>); /* <remark>condvar_broadcast(cv);</remark> */
<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting></para>
 
<para>The wait operation, <emphasis>condvar_wait_timeout</emphasis>,
always puts the calling thread to sleep. The thread then sleeps until
another thread invokes <emphasis>condvar_broadcast</emphasis> on the
same condition variable or until it is woken up by
<emphasis>condvar_signal</emphasis>. The
<emphasis>condvar_signal</emphasis> operation unblocks the first thread
blocking on the condition variable while the
<emphasis>condvar_broadcast</emphasis> operation unblocks all threads
blocking there. If there are no blocking threads, these two operations
have no efect.</para>
<para>The wait operation, <code>condvar_wait_timeout</code>, always puts
the calling thread to sleep. The thread then sleeps until another thread
invokes <code>condvar_broadcast</code> on the same condition variable or
until it is woken up by <code>condvar_signal</code>. The
<code>condvar_signal</code> operation unblocks the first thread blocking
on the condition variable while the <code>condvar_broadcast</code>
operation unblocks all threads blocking there. If there are no blocking
threads, these two operations have no efect.</para>
 
<para>Note that the threads must synchronize over a dedicated mutex. To
prevent race condition between <emphasis>condvar_wait_timeout</emphasis>
and <emphasis>condvar_signal</emphasis> or
<emphasis>condvar_broadcast</emphasis>, the mutex is passed to
<emphasis>condvar_wait_timeout</emphasis> which then atomically puts the
calling thread asleep and unlocks the mutex. When the thread eventually
wakes up, <emphasis>condvar_wait</emphasis> regains the mutex and
prevent race condition between <code>condvar_wait_timeout</code> and
<code>condvar_signal</code> or <code>condvar_broadcast</code>, the mutex
is passed to <code>condvar_wait_timeout</code> which then atomically
puts the calling thread asleep and unlocks the mutex. When the thread
eventually wakes up, <code>condvar_wait</code> regains the mutex and
returns.</para>
 
<para>Also note, that there is no conditional operation for condition
variables. Such an operation would make no sence since condition
variables are defined to forget events for which there is no waiting
thread and because <emphasis>condvar_wait</emphasis> must always go to
sleep. The operation with timeout is supported as usually.</para>
thread and because <code>condvar_wait</code> must always go to sleep.
The operation with timeout is supported as usually.</para>
 
<para>In HelenOS, condition variables are based on wait queues. As it is
already mentioned above, wait queues remember missed events while
346,55 → 338,53
condition variables are designed for scenarios in which an event might
occur very many times without being picked up by any waiting thread. On
the other hand, wait queues would remember any event that had not been
picked up by a call to <emphasis>waitq_sleep_timeout</emphasis>.
Therefore, if wait queues were used directly and without any changes to
implement condition variables, the missed_wakeup counter would hurt
performance of the implementation: the <code>while</code> loop in
<emphasis>condvar_wait_timeout</emphasis> would effectively do busy
waiting until all missed wakeups were discarded.</para>
picked up by a call to <code>waitq_sleep_timeout</code>. Therefore, if
wait queues were used directly and without any changes to implement
condition variables, the missed_wakeup counter would hurt performance of
the implementation: the <code>while</code> loop in
<code>condvar_wait_timeout</code> would effectively do busy waiting
until all missed wakeups were discarded.</para>
 
<para>The requirement on the wait operation to atomically put the caller
to sleep and release the mutex poses an interesting problem on
<emphasis>condvar_wait_timeout</emphasis>. More precisely, the thread
should sleep in the condvar's wait queue prior to releasing the mutex,
but it must not hold the mutex when it is sleeping.</para>
<code>condvar_wait_timeout</code>. More precisely, the thread should
sleep in the condvar's wait queue prior to releasing the mutex, but it
must not hold the mutex when it is sleeping.</para>
 
<para>Problems described in the two previous paragraphs are addressed in
HelenOS by dividing the <emphasis>waitq_sleep_timeout</emphasis>
function into three pieces:</para>
HelenOS by dividing the <code>waitq_sleep_timeout</code> function into
three pieces:</para>
 
<orderedlist>
<listitem>
<para><emphasis>waitq_sleep_prepare</emphasis> prepares the thread
to go to sleep by, among other things, locking the wait
queue;</para>
<para><code>waitq_sleep_prepare</code> prepares the thread to go to
sleep by, among other things, locking the wait queue;</para>
</listitem>
 
<listitem>
<para><emphasis>waitq_sleep_timeout_unsafe</emphasis> implements the
core blocking logic;</para>
<para><code>waitq_sleep_timeout_unsafe</code> implements the core
blocking logic;</para>
</listitem>
 
<listitem>
<para><emphasis>waitq_sleep_finish</emphasis> performs cleanup after
<emphasis>waitq_sleep_timeout_unsafe</emphasis>; after this call,
the wait queue spinlock is guaranteed to be unlocked by the
caller</para>
<para><code>waitq_sleep_finish</code> performs cleanup after
<code>waitq_sleep_timeout_unsafe</code>; after this call, the wait
queue spinlock is guaranteed to be unlocked by the caller</para>
</listitem>
</orderedlist>
 
<para>The stock <emphasis>waitq_sleep_timeout</emphasis> is then a mere
wrapper that calls these three functions. It is provided for convenience
in cases where the caller doesn't require such a low level control.
However, the implementation of <emphasis>condvar_wait_timeout</emphasis>
does need this finer-grained control because it has to interleave calls
to these functions by other actions. It carries its operations out in
the following order:</para>
<para>The stock <code>waitq_sleep_timeout</code> is then a mere wrapper
that calls these three functions. It is provided for convenience in
cases where the caller doesn't require such a low level control.
However, the implementation of <code>condvar_wait_timeout</code> does
need this finer-grained control because it has to interleave calls to
these functions by other actions. It carries its operations out in the
following order:</para>
 
<orderedlist>
<listitem>
<para>calls <emphasis>waitq_sleep_prepare</emphasis> in order to
lock the condition variable's wait queue,</para>
<para>calls <code>waitq_sleep_prepare</code> in order to lock the
condition variable's wait queue,</para>
</listitem>
 
<listitem>
406,7 → 396,7
</listitem>
 
<listitem>
<para>calls <emphasis>waitq_sleep_timeout_unsafe</emphasis>,</para>
<para>calls <code>waitq_sleep_timeout_unsafe</code>,</para>
</listitem>
 
<listitem>
414,7 → 404,7
</listitem>
 
<listitem>
<para>calls <emphasis>waitq_sleep_finish</emphasis>.</para>
<para>calls <code>waitq_sleep_finish</code>.</para>
</listitem>
</orderedlist>
</section>