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<chapter id="sync">

  <?dbhtml filename="sync.html"?>

  <title>Mutual exclusion and synchronization</title>

  <section>

    <title>Introduction</title>

    <para>The HelenOS operating system is designed to make use of the

    parallelism offered by the hardware and to exploit concurrency of both the

    kernel and userspace tasks. This is achieved through multiprocessor

    support and several levels of multiprogramming such as multitasking,

    multithreading and also through userspace pseudo threads. However, such a

    highly concurrent environment needs safe and efficient ways to handle

    mutual exclusion and synchronization of many execution flows.</para>

  </section>

  <section>

    <title>Active kernel primitives</title>

    <section>

      <title>Spinlocks</title>

      <para>The basic mutual exclusion primitive is the spinlock. The spinlock

      implements active waiting for the availability of a memory lock (i.e.

      simple variable) in a multiprocessor-safe manner. This safety is

      achieved through the use of a specialized, architecture-dependent,

      atomic test-and-set operation which either locks the spinlock (i.e. sets

      the variable) or, provided that it is already locked, leaves it

      unaltered. In any case, the test-and-set operation returns a value, thus

      signalling either success (i.e. zero return value) or failure (i.e.

      non-zero value) in acquiring the lock. Note that this makes a

      fundamental difference between the naive algorithm that doesn't use the

      atomic operation and the spinlock algortihm. While the naive algorithm

      is prone to race conditions on SMP configurations and thus is completely

      SMP-unsafe, the spinlock algorithm eliminates the possibility of race

      conditions and is suitable for mutual exclusion use.</para>

      <para>The semantics of the test-and-set operation is that the spinlock

      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>

      <para>Nevertheless, there is a special issue related to hardware

      optimizations that modern processors implement. Particularly problematic

      is the out-of-order execution of instructions within the critical

      section protected by a spinlock. The processors are always

      self-consistent so that they can carry out speculatively executed

      instructions in the right order with regard to dependencies among those

      instructions. However, the dependency between instructions inside the

      critical section and those that implement locking and unlocking of the

      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>

      <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.

      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

      preemption is priority inversion problem avoidance. For the same reason,

      threads are strongly discouraged from sleeping when they hold a

      spinlock.</para>

      <para>To summarize, spinlocks represent very simple and essential mutual

      exclusion primitive for SMP systems. On the other hand, spinlocks scale

      poorly because of the active loop they are based on. Therefore,

      spinlocks are used in HelenOS only for short-time mutual exclusion and

      in cases where the mutual exclusion is required out of thread context.

      Lastly, spinlocks are used in the construction of passive

      synchronization primitives.</para>

    </section>

  </section>

  <section>

    <title>Passive kernel synchronization</title>

    <section>

      <title>Wait queues</title>

      <para>A wait queue is the basic passive synchronization primitive on

      which all other passive synchronization primitives are built. Simply

      put, it allows a thread to sleep until an event associated with the

      particular wait queue occurs. Multiple threads are notified about

      incoming events in a first come, first served fashion. Moreover, should

      the event come before any thread waits for it, it is recorded in the

      wait queue as a missed wakeup and later forwarded to the first thread

      that decides to wait in the queue. The inner structures of the wait

      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>

      <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

          threads;</para>

        </listitem>

        <listitem>

          <para>another thread calls

          <emphasis>waitq_interrupt_sleep</emphasis> on the sleeping

          thread;</para>

        </listitem>

        <listitem>

          <para>the sleep times out provided that none of the previous

          occurred within a specified time limit; the limit can be

          infinity.</para>

        </listitem>

      </orderedlist>

      <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>

      <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>

    </section>

    <section>

      <title>Semaphores</title>

      <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>

      <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>

    </section>

    <section>

      <title>Mutexes</title>

      <para>Mutexes are sometimes referred to as binary sempahores. That means

      that mutexes are like semaphores that allow only one thread in its

      critical section. Indeed, mutexes in HelenOS are implemented exactly in

      this way: they are built on top of semaphores. From another point of

      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>

    </section>

    <section>

      <title>Reader/writer locks</title>

      <para>Reader/writer locks, or rwlocks, are by far the most complicated

      synchronization primitive within the kernel. The goal of these locks is

      to improve concurrency of applications, in which threads need to

      synchronize access to a shared resource, and that access can be

      partitioned into a read-only mode and a write mode. Reader/writer locks

      should make it possible for several, possibly many, readers to enter the

      critical section, provided that no writer is currently in the critical

      section, or to be in the critical section contemporarily. Writers are

      allowed to enter the critical section only individually, provided that

      no reader is in the critical section already. Applications, in which the

      majority of operations can be done in the read-only mode, can benefit

      from increased concurrency created by reader/writer locks.</para>

      <para>During reader/writer lock construction, a decision should be made

      whether readers will be prefered over writers or whether writers will be

      prefered over readers in cases when the lock is not currently held and

      both a reader and a writer want to gain the lock. Some operating systems

      prefer one group over the other, creating thus a possibility for

      starving the unprefered group. In the HelenOS operating system, none of

      the two groups is prefered. The lock is granted on a first come, first

      served basis with the additional note that readers are granted the lock

      in the biggest possible batch.</para>

      <para>With this policy and the timeout modes of operation, the direct

      hand-off becomes much more complicated. For instance, a writer leaving

      the critical section must wake up all leading readers in the rwlock's

      wait queue or one leading writer or no-one if no thread is waiting.

      Similarily, the last reader leaving the critical section must wakeup the

      sleeping writer if there are any sleeping threads left at all. As

      another example, if a writer at the beginning of the rwlock's wait queue

      times out and the lock is held by at least one reader, the writer which

      has timed out must first wake up all readers that follow him in the

      queue prior to signalling the timeout itself and giving up.</para>

      <para>Due to the issues mentioned in the previous paragraph, the

      reader/writer lock imlpementation needs to walk the rwlock wait queue's

      list of sleeping threads directly, in order to find out the type of

      access that the queueing threads demand. This makes the code difficult

      to understand and dependent on the internal implementation of the wait

      queue. Nevertheless, it remains unclear to the authors of HelenOS

      whether a simpler but equivalently fair solution exists.</para>

      <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.

      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

      procedure outlined above.</para>

      <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>

      <orderedlist>

        <listitem>

          <para>there are other readers in the critical section and</para>

        </listitem>

        <listitem>

          <para>there are no sleeping threads waiting for the exclusive

          mutex.</para>

        </listitem>

      </orderedlist>

      <para>If both conditions are true, the reader will bypass the mutex,

      increment the number of readers in the critical section and then enter

      the critical section. Note that if there are any sleeping threads at the

      beginning of the wait queue, the first must be a writer. If the

      conditions are not fulfilled, the reader normally waits until the

      exclusive mutex is granted to it.</para>

    </section>

    <section>

      <title>Condition variables</title>

      <para>Condition variables can be used for waiting until a condition

      becomes true. In this respect, they are similar to wait queues. But

      contrary to wait queues, condition variables have different semantics

      that allows events to be lost when there is no thread waiting for them.

      In order to support this, condition variables don't use direct hand-off

      and operate in a way similar to the example below. A thread waiting for

      the condition becoming true does the following:</para>

      <para><programlisting language="C"><function>mutex_lock</function>(<varname>mtx</varname>);

while (!<varname>condition</varname>)

        <function>condvar_wait_timeout</function>(<varname>cv</varname>, <varname>mtx</varname>);

/* <remark>the condition is true, do something</remark> */

<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting></para>

      <para>A thread that causes the condition become true signals this event

      like this:</para>

      <para><programlisting><function>mutex_lock</function>(<varname>mtx</varname>);

<varname>condition</varname> = <constant>true</constant>;

<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>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

      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>

      <para>In HelenOS, condition variables are based on wait queues. As it is

      already mentioned above, wait queues remember missed events while

      condition variables must not do so. This is reasoned by the fact that

      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>

      <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>

      <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>

      <orderedlist>

        <listitem>

          <para><emphasis>waitq_sleep_prepare</emphasis> 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>

        </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>

        </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>

      <orderedlist>

        <listitem>

          <para>calls <emphasis>waitq_sleep_prepare</emphasis> in order to

          lock the condition variable's wait queue,</para>

        </listitem>

        <listitem>

          <para>releases the mutex,</para>

        </listitem>

        <listitem>

          <para>clears the counter of missed wakeups,</para>

        </listitem>

        <listitem>

          <para>calls <emphasis>waitq_sleep_timeout_unsafe</emphasis>,</para>

        </listitem>

        <listitem>

          <para>retakes the mutex,</para>

        </listitem>

        <listitem>

          <para>calls <emphasis>waitq_sleep_finish</emphasis>.</para>

        </listitem>

      </orderedlist>

    </section>

  </section>

  <section>

    <title>Userspace synchronization</title>

    <section>

      <title>Futexes</title>

      <para></para>

    </section>

  </section>

</chapter>