Kernel: Mutex - Revision history

Raion at 23:54, 9 January 2026

2026-01-09T23:54:26Z

← Older revision		Revision as of 23:54, 9 January 2026
Line 89:		Line 89:

	Porting: illumos adaptive mutex closest match. Differences in turnstile priority blocking vs IRIX explicit inheritance chains.		Porting: illumos adaptive mutex closest match. Differences in turnstile priority blocking vs IRIX explicit inheritance chains.

			[[Category: Kernel Documentation]]

Raion: /* Similarities to illumos */

2026-01-04T03:24:39Z

Similarities to illumos

← Older revision		Revision as of 03:24, 4 January 2026
Line 73:		Line 73:
	Threshold wake (sv_threshold).		Threshold wake (sv_threshold).

	== Similarities to illumos ~~and BSD Kernel Implementations~~ ==		== Similarities to illumos Implementation ==
	illumos (Solaris-derived)		illumos (Solaris-derived)

	Strong similarity due to shared SVR4 heritage:		Strong similarity due to shared SVR4 heritage:

	Adaptive mutexes with priority inheritance (similar owner word, waiter lists).		Adaptive mutexes with priority inheritance (similar owner word, waiter lists).

	mutex_enter, mutex_tryenter, mutex_exit.		mutex_enter, mutex_tryenter, mutex_exit.

	Turnstiles for sleeping/waiting (similar to sv_t but more integrated).		Turnstiles for sleeping/waiting (similar to sv_t but more integrated).

	CVs and semaphores separate but similar.		CVs and semaphores separate but similar.

	Metering via kstat or dtrace.		Metering via kstat or dtrace.

	Porting: illumos adaptive mutex closest match. Differences in turnstile priority blocking vs IRIX explicit inheritance chains.		Porting: illumos adaptive mutex closest match. Differences in turnstile priority blocking vs IRIX explicit inheritance chains.

Raion at 03:23, 4 January 2026

2026-01-04T03:23:40Z

← Older revision		Revision as of 03:23, 4 January 2026
Line 1:		Line 1:
	== Overview ==		== Overview ==
	The IRIX kernel mutex implementation provides basic mutual exclusion primitives with priority inheritance to mitigate priority inversion, atomic owner tracking via compare-and-swap, optional metering/statistics, and integration with the broader ksync (kernel synchronization) framework that includes sv_t (synchronization variables) for semaphores, condition variables, and other waitable objects.		The IRIX kernel mutex implementation provides basic mutual exclusion primitives with priority inheritance to mitigate priority inversion, atomic owner tracking via compare-and-swap, optional metering/statistics, and integration with the broader [[Kernel:ksync]] (kernel synchronization) framework that includes sv_t (synchronization variables) for semaphores, condition variables, and other waitable objects.
	The design uses a single-word owner field (m_bits) combining owner thread pointer with flags (lock bit, queue bit). Contention spins briefly then sleeps on a circular doubly-linked waiter queue. Priority inheritance propagates up blocked chains. The implementation supports debugging/metering via optional info structures and integrates with spinlocks, bitlocks, mrlocks, and semaphores through sv_t wrappers.		The design uses a single-word owner field (m_bits) combining owner thread pointer with flags (lock bit, queue bit). Contention spins briefly then sleeps on a circular doubly-linked waiter queue. Priority inheritance propagates up blocked chains. The implementation supports debugging/metering via optional info structures and integrates with spinlocks, bitlocks, mrlocks, and semaphores through sv_t wrappers.
	== Key Functions ==		== Key Functions ==

Raion: Created page with "== Overview == The IRIX kernel mutex implementation provides basic mutual exclusion primitives with priority inheritance to mitigate priority inversion, atomic owner tracking via compare-and-swap, optional metering/statistics, and integration with the broader ksync (kernel synchronization) framework that includes sv_t (synchronization variables) for semaphores, condition variables, and other waitable objects. The design uses a single-word owner field (m_bits) combining o..."

2026-01-04T03:23:07Z

Created page with "== Overview == The IRIX kernel mutex implementation provides basic mutual exclusion primitives with priority inheritance to mitigate priority inversion, atomic owner tracking via compare-and-swap, optional metering/statistics, and integration with the broader ksync (kernel synchronization) framework that includes sv_t (synchronization variables) for semaphores, condition variables, and other waitable objects. The design uses a single-word owner field (m_bits) combining o..."

New page

== Overview ==
The IRIX kernel mutex implementation provides basic mutual exclusion primitives with priority inheritance to mitigate priority inversion, atomic owner tracking via compare-and-swap, optional metering/statistics, and integration with the broader ksync (kernel synchronization) framework that includes sv_t (synchronization variables) for semaphores, condition variables, and other waitable objects.
The design uses a single-word owner field (m_bits) combining owner thread pointer with flags (lock bit, queue bit). Contention spins briefly then sleeps on a circular doubly-linked waiter queue. Priority inheritance propagates up blocked chains. The implementation supports debugging/metering via optional info structures and integrates with spinlocks, bitlocks, mrlocks, and semaphores through sv_t wrappers.
== Key Functions ==
Initialization (kmutei_init)
Creates zones for mutex_t and sv_t structures. Initializes hash buckets for optional wchan info (debug/metering).
Allocation (mutex_alloc / init_mutex)
Zone-allocates mutex_t, clears bits, optionally allocates metering/info structure.
Deallocation (mutex_destroy / mutex_dealloc)
Frees info structure, zone-frees mutex.
Core Operations

mutex_trylock: Atomic compare-and-swap to set owner; updates metering.
mutex_lock: Trylock → on miss, queue and sleep with priority inheritance probing.
mutex_unlock: Clears owner; if waiters, wakes highest-priority waiter (single thread).
mutex_wait: Queues thread, handles inheritance, blocks (non-breakable).
mutex_wake: Dequeues highest-priority waiter, transfers ownership, updates metering.

Priority Inheritance

resource_raise_owner_pri: Boosts owner priority to waiter’s.
resource_test_owner_pri: Detects inversion, raises or spins/yields.
drop_inheritance: Restores base priority on unlock.
Supports chained inheritance via k_inherit pointer.

Waiter Queue Management

Circular doubly-linked list in m_queue.
Queued in approximate priority order (mutex_queue).
Single waiter woken on unlock (mutex_wake).

Synchronization Variables (sv_t)
General-purpose wait queue supporting:

FIFO, LIFO, priority, keyed ordering.
Signal (wake one), broadcast (wake all), bounded broadcast.
Timed waits, signal-breakable waits.
Integration with mutex/spin/bit/mrlock/sema release.

== Undocumented or IRIX-Specific Interfaces and Behaviors ==
Critical Structures

mutex_t:
__psunsigned_t m_bits — owner thread pointer + flags (low bit lock, next bit queue).
kthread_t *m_queue — circular waiter list head.
Optional m_info pointer for metering/wchan info.

sv_t:
__psunsigned_t sv_queue — waiter pointer + lock bit + type (FIFO/LIFO/PRIO/KEYED).
Optional sv_info for metering.

Flags/macros:
MUTEX_LOCKBIT, MUTEX_QUEUEBIT
SV_LOCK, SV_TYPE masks
KT_WMUTEX, KT_WSV thread sleep flags

Priority Inheritance Details

Aggressive: propagates through chains, handles indirect waits.
Temporary priority boost with KT_PS flag.
Supports real-time and timeshare scheduling.

Metering and Debug (SEMAMETER/SEMAINFO)

Optional per-object statistics (lock attempts, hold/wait times, contention).
Hash bucket lists for idbg scanning.

sv_t Flexibility

Unified queue for multiple lock types via wrapper functions.
Supports bounded broadcast (wakes only original waiter set).
Threshold wake (sv_threshold).

== Similarities to illumos and BSD Kernel Implementations ==
illumos (Solaris-derived)
Strong similarity due to shared SVR4 heritage:

Adaptive mutexes with priority inheritance (similar owner word, waiter lists).
mutex_enter, mutex_tryenter, mutex_exit.
Turnstiles for sleeping/waiting (similar to sv_t but more integrated).
CVs and semaphores separate but similar.
Metering via kstat or dtrace.

Porting: illumos adaptive mutex closest match. Differences in turnstile priority blocking vs IRIX explicit inheritance chains.