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preempt_disable() and local_irq_disable/save() are in principle per CPU big
kernel locks. This has several downsides:
- The protection scope is unknown
- Violation of protection rules is hard to detect by instrumentation
- For PREEMPT_RT such sections, unless in low level critical code, can
violate the preemptability constraints.
To address this PREEMPT_RT introduced the concept of local_locks which are
strictly per CPU.
The lock operations map to preempt_disable(), local_irq_disable/save() and
the enabling counterparts on non RT enabled kernels.
If lockdep is enabled local locks gain a lock map which tracks the usage
context. This will catch cases where an area is protected by
preempt_disable() but the access also happens from interrupt context. local
locks have identified quite a few such issues over the years, the most
recent example is:
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|---|---|---|
| .. | ||
| index.rst | ||
| lockdep-design.rst | ||
| lockstat.rst | ||
| locktorture.rst | ||
| locktypes.rst | ||
| mutex-design.rst | ||
| rt-mutex-design.rst | ||
| rt-mutex.rst | ||
| spinlocks.rst | ||
| ww-mutex-design.rst | ||