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Hardens `library/cpp/yt/rseq` for the case where it is linked into a dlopen'd,
position-independent module (e.g. a YQL UDF `.so`). Extracted from the profiling
work that enables the rseq fast path by default.
**TLS model.** The weak `__rseq_abi` gets `global-dynamic` linkage under
`__PIC__/__PIE__` (`initial-exec` otherwise), mirroring `contrib/libs/tcmalloc`.
`initial-exec` needs a slot in the static TLS block reserved at startup, which
the loader cannot grant a module dlopen'd later — the module would fail to load
with "cannot allocate memory in static TLS block". This only changes the cold
`&__rseq_abi` accesses; the hot path still reads `*(thread_pointer + CpuIdFieldOffset)`.
**Runtime safety probe `IsPerCpuFastPathSafe()`.** The cached thread-pointer
offset is valid only when `__rseq_abi` sits at a fixed offset from the thread
pointer — a glibc-owned area or the static TLS block (incl. tcmalloc), the common
case. When our `__rseq_abi` instead lands in a dlopen'd module's *dynamically
allocated* TLS, the offset is valid only on the thread that computed it; on other
threads the hot path's first store (`area->rseq_cs`) would corrupt unrelated
memory. The probe spawns one thread and checks — by pointer comparison, never
dereferencing the suspect offset — that the offset names that thread's rseq area;
if not, callers use the atomic fallback. Decided once and cached (one thread spawn
at first use).=
commit_hash:633f58f500d9d097800da81f526c56283445ffc7
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Self-contained current-CPU-id reader backed by Linux **rseq** (restartable
sequences), with **no third-party dependency** (no librseq):
* The rseq ABI is hand-defined; the calling thread is registered lazily via the
rseq syscall.
* Fast path is a single inlined, **branch-free** thread-local read. The offset
always points at a readable `cpu_id` -- the glibc-owned area when glibc registers
rseq (>= 2.35, via the weak `__rseq_offset`/`__rseq_size`), otherwise our own
area -- so an unregistered thread reads `-1` and routes to the slow path.
* Falls back to `sched_getcpu()` (Linux) or `0` (darwin/windows). Works on glibc
**and musl** alike (librseq does not build on musl).
Fiber-TLS contract: the inlined read must be reached only via a non-inlinable,
fiber-switch-free frame (a virtual call or `YT_PREVENT_TLS_CACHING`).
#### Benchmark -- cost of one cpu-id read
| source | time / call |
|---|---|
| `GetCurrentCpuId()` (rseq) | **0.34 ns** |
| `sched_getcpu()` (vDSO) | 3.5 ns |
| `rdtscp` (what `TTscp::Get()` does) | 23 ns |
This is an alternative to the librseq-based review/13886037 -- same speed, but no
contrib dependency and it also covers musl. The unit test pins to each allowed CPU
and asserts the reported id matches.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
commit_hash:09d282c2f48755836b1cd68cedbffc3c6a662eed
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