Documentation/locking/percpu-rw-semaphore.rst
Source file repositories/reference/linux-study-clean/Documentation/locking/percpu-rw-semaphore.rst
File Facts
- System
- Linux kernel
- Corpus path
Documentation/locking/percpu-rw-semaphore.rst- Extension
.rst- Size
- 1150 bytes
- Lines
- 29
- Domain
- Support Tooling And Documentation
- Bucket
- Documentation
- Inferred role
- Support Tooling And Documentation: documentation
- Status
- atlas-only
Why This File Exists
Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.
Dependency Surface
- No C-style include directives detected by the generator.
Detected Declarations
- No top-level syscall, struct, function, initcall, or export declaration detected by the generator.
Annotated Snippet
====================
Percpu rw semaphores
====================
Percpu rw semaphores is a new read-write semaphore design that is
optimized for locking for reading.
The problem with traditional read-write semaphores is that when multiple
cores take the lock for reading, the cache line containing the semaphore
is bouncing between L1 caches of the cores, causing performance
degradation.
Locking for reading is very fast, it uses RCU and it avoids any atomic
instruction in the lock and unlock path. On the other hand, locking for
writing is very expensive, it calls synchronize_rcu() that can take
hundreds of milliseconds.
The lock is declared with "struct percpu_rw_semaphore" type.
The lock is initialized with percpu_init_rwsem, it returns 0 on success
and -ENOMEM on allocation failure.
The lock must be freed with percpu_free_rwsem to avoid memory leak.
The lock is locked for read with percpu_down_read, percpu_up_read and
for write with percpu_down_write, percpu_up_write.
The idea of using RCU for optimized rw-lock was introduced by
Eric Dumazet <eric.dumazet@gmail.com>.
The code was written by Mikulas Patocka <mpatocka@redhat.com>Annotation
- Atlas domain: Support Tooling And Documentation / Documentation.
- Implementation status: atlas-only.
Implementation Notes
- This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
- Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
- Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.