hujun260 opened a new pull request, #18210:
URL: https://github.com/apache/nuttx/pull/18210
## Summary
This PR contains two related improvements to the reader-writer semaphore
implementation:
1. Optimize `up_read()` and `up_write()` APIs to reduce unnecessary context
switches
2. Refactor control flow in `up_read()` to eliminate goto statements and
achieve
MISRA HIS compliance
## Motivation and Problem
### Part 1: Performance Optimization
The original implementation had redundant context switch operations in the
reader-writer semaphore release paths. The `up_wait()` function was being
called
multiple times in different code paths, creating unnecessary wake-ups and
context
switches even when no waiters were present, impacting system performance.
### Part 2: MISRA HIS Compliance
The `up_read()` function used goto-based control flow which violates MISRA
HIS
coding standards for safety-critical systems. Structured if-else blocks
improve
code clarity, verifiability, and compliance with automotive safety standards.
## Changes
### Commit 1: Performance Optimization
- Consolidate `up_wait()` calls to reduce unnecessary context switches
- Move `up_wait()` only to paths where waiters actually exist
- Simplify reader count decrement logic with combined condition check
### Commit 2: MISRA HIS Compliance
- Replace goto statements with structured if-else blocks in `up_read()`
- Maintain identical functional behavior with improved code structure
- Single exit point after all conditional paths
## Impact
- **Performance**: Reduced context switch overhead in high-contention
scenarios
- **Compliance**: Achieves MISRA HIS compliance for goto elimination metrics
- **Code Quality**: Improved readability with structured control flow
- **Verifiability**: Single-exit paths improve static analysis
- **Backward Compatibility**: No functional changes; identical runtime
semantics
- **Latency**: Reduced interrupt disable time from fewer unnecessary wake-ups
## Verification
- [x] Code compiles without warnings on ARM GCC 10.x
- [x] Verified on QEMU ARMv7-A simulator with multimedia profile
- [x] Reader-writer semaphore operations verified:
- Single reader access patterns
- Single writer access patterns
- Mixed reader-writer contention scenarios
- Waiter notification behavior verified
- No unnecessary context switches on optimized paths
- [x] Static analysis shows improved compliance metrics
## Testing
Tested with:
- ARM GCC 10.x compiler
- QEMU ARMv7-A simulation
- Reader-writer semaphore scenarios:
- Multiple readers with no writers
- Multiple writers (exclusive access)
- Mixed reader-writer access patterns
- Waiter wake-up correctness
- Lock holder tracking
- Context switch reduction verification
## Related Issues
Addresses performance optimization for high-contention scenarios and MISRA
HIS
compliance for goto elimination.
Reference: https://github.com/apache/nuttx/pull/17909
## Files Changed
- `sched/semaphore/sem_rw.c` (23 lines total: 10 insertions, 13 deletions
across 2 commits)
- Commit 1: 8 lines (3 insertions, 5 deletions) - Performance optimization
- Commit 2: 15 lines (7 insertions, 8 deletions) - HIS_GOTO compliance
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