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=====Sleeping Spinlocks=====

Spinlocks are used in the kernel to guarantee only one thread at a time can
be executing within a protected section of code. Their advantages over mutex locks
are simplicity and speed of execution. However, if the lock is held too long
or the odds of contention for it from other threads of execution becomes
significant, spinlocks can become a performance bottleneck and a source of
latency. Of course latency is a particularly important issue in real-time systems.

In order to reduce latency the PREEMPT_RT patch converts the majority of kernel
spinlocks into rt_mutex locks. The underlying changes necessary to support this have mostly
been upstreamed already. These include changes to reduce the scope of
code executed with a lock held, and the addition of a new type to distinguish
true spinlocks from the rt_mutex used inside the modified spinlock_t type.
In order to minimize the changes to the
kernel source the existing spinlock_t datatype and the functions which operate
on it retain their old names but, when PREEMPT_RT is enabled, now refer to an rt_mutex lock.
Unfortunately this is somewhat confusing when reading the code, since a
spinlock_t is not really a spinlock when PREEMPT_RT is enabled.

Since sleeping spinlocks obviously need to allow preemption they cannot
be locked while preemption or interrupts are disabled. If this is necessary
a traditional spinlock must be used.

When an rt_mutex lock cannot be used for some reason the
code must be changed to use the new raw_spinlock_t, which forces
traditional spinlock behavior. It is also necessary to use the corresponding
raw_* variants of the spinlock functions when operating on this newer datatype.
Some older documentation on PREEMPT_RT states that the old function names are
used with the new datatype and the actual lock type is detected automatically,
but this is not currently the case. The datatype and functions used to operate
on it must match.

====See also:====

McKenney, Paul E. (8/2005). A Realtime Preemption Overview. LWN.net, Articles. Retrieved from https://lwn.net/Articles/146861

Sven-Thorsten Dietrich, Daniel Walker (2005). The Evolution of Real-Time Linux. Open Source Automation Development Lab. Retrieved from https://www.osadl.org/fileadmin/events/rtlws-2005/SvenThorstenDietrich.pdf

Rostedt, Steven (2015). The RT Patch. Retrieved from http://events.linuxfoundation.org/sites/events/files/slides/linux-con-rt-into-mainline-2015.pdf
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