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>Spinlocks</TITLE
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><H1
><A
NAME="KERNEL-SPINLOCKS">Spinlocks</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN1693"
></A
><H2
>Name</H2
>cyg_spinlock_create, cyg_spinlock_destroy, cyg_spinlock_spin, cyg_spinlock_clear, cyg_spinlock_test, cyg_spinlock_spin_intsave, cyg_spinlock_clear_intsave -- Low-level Synchronization Primitive</DIV
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>Synopsis</H2
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/kernel/kapi.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_init</CODE
>(cyg_spinlock_t* lock, cyg_bool_t locked);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_destroy</CODE
>(cyg_spinlock_t* lock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_spin</CODE
>(cyg_spinlock_t* lock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_clear</CODE
>(cyg_spinlock_t* lock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_bool_t cyg_spinlock_try</CODE
>(cyg_spinlock_t* lock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_bool_t cyg_spinlock_test</CODE
>(cyg_spinlock_t* lock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_spin_intsave</CODE
>(cyg_spinlock_t* lock, cyg_addrword_t* istate);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_spinlock_clear_intsave</CODE
>(cyg_spinlock_t* lock, cyg_addrword_t istate);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-SPINLOCKS-DESCRIPTION"
></A
><H2
>Description</H2
><P
>Spinlocks provide an additional synchronization primitive for
applications running on SMP systems. They operate at a lower level
than the other primitives such as mutexes, and for most purposes the
higher-level primitives should be preferred. However there are some
circumstances where a spinlock is appropriate, especially when
interrupt handlers and threads need to share access to hardware, and
on SMP systems the kernel implementation itself depends on spinlocks.
</P
><P
>Essentially a spinlock is just a simple flag. When code tries to claim
a spinlock it checks whether or not the flag is already set. If not
then the flag is set and the operation succeeds immediately. The exact
implementation of this is hardware-specific, for example it may use a
test-and-set instruction to guarantee the desired behaviour even if
several processors try to access the spinlock at the exact same time.
If it is not possible to claim a spinlock then the current thead spins
in a tight loop, repeatedly checking the flag until it is clear. This
behaviour is very different from other synchronization primitives such
as mutexes, where contention would cause a thread to be suspended. The
assumption is that a spinlock will only be held for a very short time.
If claiming a spinlock could cause the current thread to be suspended
then spinlocks could not be used inside interrupt handlers, which is
not acceptable.
</P
><P
>This does impose a constraint on any code which uses spinlocks.
Specifically it is important that spinlocks are held only for a short
period of time, typically just some dozens of instructions. Otherwise
another processor could be blocked on the spinlock for a long time,
unable to do any useful work. It is also important that a thread which
owns a spinlock does not get preempted because that might cause
another processor to spin for a whole timeslice period, or longer. One
way of achieving this is to disable interrupts on the current
processor, and the function
<TT
CLASS="FUNCTION"
>cyg_spinlock_spin_intsave</TT
> is provided to
facilitate this.
</P
><P
>Spinlocks should not be used on single-processor systems. Consider a
high priority thread which attempts to claim a spinlock already held
by a lower priority thread: it will just loop forever and the lower
priority thread will never get another chance to run and release the
spinlock. Even if the two threads were running at the same priority,
the one attempting to claim the spinlock would spin until it was
timesliced and a lot of cpu time would be wasted. If an interrupt
handler tried to claim a spinlock owned by a thread, the interrupt
handler would loop forever. Therefore spinlocks are only appropriate
for SMP systems where the current owner of a spinlock can continue
running on a different processor.
</P
><P
>Before a spinlock can be used it must be initialized by a call to
<TT
CLASS="FUNCTION"
>cyg_spinlock_init</TT
>. This takes two arguments, a
pointer to a <TT
CLASS="FUNCTION"
>cyg_spinlock_t</TT
> data structure, and
a flag to specify whether the spinlock starts off locked or unlocked.
If a spinlock is no longer required then it can be destroyed by a call
to <TT
CLASS="FUNCTION"
>cyg_spinlock_destroy</TT
>.
</P
><P
>There are two routines for claiming a spinlock:
<TT
CLASS="FUNCTION"
>cyg_spinlock_spin</TT
> and
<TT
CLASS="FUNCTION"
>cyg_spinlock_spin_intsave</TT
>. The former can be used
when it is known the current code will not be preempted, for example
because it is running in an interrupt handler or because interrupts
are disabled. The latter will disable interrupts in addition to
claiming the spinlock, so is safe to use in all circumstances. The
previous interrupt state is returned via the second argument, and
should be used in a subsequent call to
<TT
CLASS="FUNCTION"
>cyg_spinlock_clear_intsave</TT
>.
</P
><P
>Similarly there are two routines for releasing a spinlock:
<TT
CLASS="FUNCTION"
>cyg_spinlock_clear</TT
> and
<TT
CLASS="FUNCTION"
>cyg_spinlock_clear_intsave</TT
>. Typically
the former will be used if the spinlock was claimed by a call to
<TT
CLASS="FUNCTION"
>cyg_spinlock_spin</TT
>, and the latter when
<TT
CLASS="FUNCTION"
>cyg_spinlock_intsave</TT
> was used.
</P
><P
>There are two additional routines.
<TT
CLASS="FUNCTION"
>cyg_spinlock_try</TT
> is a non-blocking version of
<TT
CLASS="FUNCTION"
>cyg_spinlock_spin</TT
>: if possible the lock will be
claimed and the function will return <TT
CLASS="LITERAL"
>true</TT
>; otherwise the function
will return immediately with failure.
<TT
CLASS="FUNCTION"
>cyg_spinlock_test</TT
> can be used to find out whether
or not the spinlock is currently locked. This function must be used
with care because, especially on a multiprocessor system, the state of
the spinlock can change at any time.
</P
><P
>Spinlocks should only be held for a short period of time, and
attempting to claim a spinlock will never cause a thread to be
suspended. This means that there is no need to worry about priority
inversion problems, and concepts such as priority ceilings and
inheritance do not apply.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-SPINLOCKS-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
>All of the spinlock functions can be called from any context,
including ISR and DSR context. Typically
<TT
CLASS="FUNCTION"
>cyg_spinlock_init</TT
> is only called during system
initialization.
</P
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