mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-06 02:24:14 +08:00
a6537be932
Add rt-mutex documentation. [rostedt@goodmis.org: Update rt-mutex-design.txt as per Randy Dunlap suggestions] Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Cc: "Randy.Dunlap" <rdunlap@xenotime.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
80 lines
3.5 KiB
Plaintext
80 lines
3.5 KiB
Plaintext
RT-mutex subsystem with PI support
|
|
----------------------------------
|
|
|
|
RT-mutexes with priority inheritance are used to support PI-futexes,
|
|
which enable pthread_mutex_t priority inheritance attributes
|
|
(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details
|
|
about PI-futexes.]
|
|
|
|
This technology was developed in the -rt tree and streamlined for
|
|
pthread_mutex support.
|
|
|
|
Basic principles:
|
|
-----------------
|
|
|
|
RT-mutexes extend the semantics of simple mutexes by the priority
|
|
inheritance protocol.
|
|
|
|
A low priority owner of a rt-mutex inherits the priority of a higher
|
|
priority waiter until the rt-mutex is released. If the temporarily
|
|
boosted owner blocks on a rt-mutex itself it propagates the priority
|
|
boosting to the owner of the other rt_mutex it gets blocked on. The
|
|
priority boosting is immediately removed once the rt_mutex has been
|
|
unlocked.
|
|
|
|
This approach allows us to shorten the block of high-prio tasks on
|
|
mutexes which protect shared resources. Priority inheritance is not a
|
|
magic bullet for poorly designed applications, but it allows
|
|
well-designed applications to use userspace locks in critical parts of
|
|
an high priority thread, without losing determinism.
|
|
|
|
The enqueueing of the waiters into the rtmutex waiter list is done in
|
|
priority order. For same priorities FIFO order is chosen. For each
|
|
rtmutex, only the top priority waiter is enqueued into the owner's
|
|
priority waiters list. This list too queues in priority order. Whenever
|
|
the top priority waiter of a task changes (for example it timed out or
|
|
got a signal), the priority of the owner task is readjusted. [The
|
|
priority enqueueing is handled by "plists", see include/linux/plist.h
|
|
for more details.]
|
|
|
|
RT-mutexes are optimized for fastpath operations and have no internal
|
|
locking overhead when locking an uncontended mutex or unlocking a mutex
|
|
without waiters. The optimized fastpath operations require cmpxchg
|
|
support. [If that is not available then the rt-mutex internal spinlock
|
|
is used]
|
|
|
|
The state of the rt-mutex is tracked via the owner field of the rt-mutex
|
|
structure:
|
|
|
|
rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1
|
|
are used to keep track of the "owner is pending" and "rtmutex has
|
|
waiters" state.
|
|
|
|
owner bit1 bit0
|
|
NULL 0 0 mutex is free (fast acquire possible)
|
|
NULL 0 1 invalid state
|
|
NULL 1 0 Transitional state*
|
|
NULL 1 1 invalid state
|
|
taskpointer 0 0 mutex is held (fast release possible)
|
|
taskpointer 0 1 task is pending owner
|
|
taskpointer 1 0 mutex is held and has waiters
|
|
taskpointer 1 1 task is pending owner and mutex has waiters
|
|
|
|
Pending-ownership handling is a performance optimization:
|
|
pending-ownership is assigned to the first (highest priority) waiter of
|
|
the mutex, when the mutex is released. The thread is woken up and once
|
|
it starts executing it can acquire the mutex. Until the mutex is taken
|
|
by it (bit 0 is cleared) a competing higher priority thread can "steal"
|
|
the mutex which puts the woken up thread back on the waiters list.
|
|
|
|
The pending-ownership optimization is especially important for the
|
|
uninterrupted workflow of high-prio tasks which repeatedly
|
|
takes/releases locks that have lower-prio waiters. Without this
|
|
optimization the higher-prio thread would ping-pong to the lower-prio
|
|
task [because at unlock time we always assign a new owner].
|
|
|
|
(*) The "mutex has waiters" bit gets set to take the lock. If the lock
|
|
doesn't already have an owner, this bit is quickly cleared if there are
|
|
no waiters. So this is a transitional state to synchronize with looking
|
|
at the owner field of the mutex and the mutex owner releasing the lock.
|