kernel - Initial commit for Existential structural tracking

Operating Systems / DragonFlyBSD - Matthew Dillon [] - 13 October 2020 01:58 UTC

- Initial commit for exislock (existance-lock) support, based on cpu tick interlocks. Requires a bit more work.

(see also previous commit 'Add missing sys/exislock.h' which wasn't actually missing at the time).

This is a type-safe critical section plus a cache-friendly soft-lock type for system data structures that allows interlocking against structural existance and usability.

The critical path has no atomic operations or cache ping-ponging. Cache ping-ponging occurs at most once per pseudo_tick (once every 2 real ticks) to reset the timeout field.

- Implements a global 'pseudo_ticks' counter. This counter is only able to count when all cpus are armed. All cpus are armed on every 1->0 transition of their mycpu->gd_exislockcnt field. This, while the interlock is held, the global pseudo_ticks counter will increment by at most one, then stall.

The cpus are disarmed when this increment occurs on even ticks and rearmed (if gd_exislockcnt is 0) on odd ticks. This the global pseudo_ticks counter increments at roughly hz / 2 under most conditions.

This means that even when the per-cpu type-safe critical section is under very heavy load, cycling constantly, the global pseudo_ticks still tends to increment on a regular basis becaues there are a lot of 1->0 transitions occurring on each cpu. Importantly, the cpus do not need to be synchronized in order for pseudo_ticks to increment.

- This codebase will be used to implement type-safe storage and lockless hash-table based caches. It works like this:

- You use exis_hold() and exis_drop() around the type-safe code sections.

While held, the global pseudo_ticks variable is guaranteed to increment no more than once (due to a prior arming condition).

- You access the hash table or other robust structural topology (with or without locks depending), then call exis_isusable() on the structure you get to determine if it is usable. If TRUE is returned, the structure will remain type-safe and will not be repurposed for the duration of the exis_hold().

You then proceed to use the structure as needed, with or without further locking depending on what you are doing. For example, accessing stat information from a vnode structure could potentially proceed without further locking.

8f7f5bd589 kernel - Initial commit for Existential structural tracking
sys/kern/kern_clock.c | 39 +++++++
sys/kern/vfs_mount.c | 1 +
sys/sys/exislock.h | 284 +++++++++++++++++++++++++++++++++++++++++++-------
sys/sys/globaldata.h | 14 ++-
4 files changed, 298 insertions(+), 40 deletions(-)


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