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25 #include <folly/Likely.h>
26 #include <folly/detail/Futex.h>
27 #include <folly/detail/MemoryIdler.h>
28 #include <folly/portability/Asm.h>
32 /// A Baton allows a thread to block once and be awoken. Captures a
33 /// single handoff, and during its lifecycle (from construction/reset
34 /// to destruction/reset) a baton must either be post()ed and wait()ed
35 /// exactly once each, or not at all.
37 /// Baton includes no internal padding, and is only 4 bytes in size.
38 /// Any alignment or padding to avoid false sharing is up to the user.
40 /// This is basically a stripped-down semaphore that supports only a
41 /// single call to sem_post and a single call to sem_wait.
43 /// The non-blocking version (MayBlock == false) provides more speed
44 /// by using only load acquire and store release operations in the
45 /// critical path, at the cost of disallowing blocking and timing out.
47 /// The current posix semaphore sem_t isn't too bad, but this provides
48 /// more a bit more speed, inlining, smaller size, a guarantee that
49 /// the implementation won't change, and compatibility with
50 /// DeterministicSchedule. By having a much more restrictive
51 /// lifecycle we can also add a bunch of assertions that can help to
52 /// catch race conditions ahead of time.
53 template <bool MayBlock = true, template <typename> class Atom = std::atomic>
55 constexpr Baton() noexcept : state_(INIT) {}
57 Baton(Baton const&) = delete;
58 Baton& operator=(Baton const&) = delete;
60 /// It is an error to destroy a Baton on which a thread is currently
61 /// wait()ing. In practice this means that the waiter usually takes
62 /// responsibility for destroying the Baton.
64 // The docblock for this function says that it can't be called when
65 // there is a concurrent waiter. We assume a strong version of this
66 // requirement in which the caller must _know_ that this is true, they
67 // are not allowed to be merely lucky. If two threads are involved,
68 // the destroying thread must actually have synchronized with the
69 // waiting thread after wait() returned. To convey causality the the
70 // waiting thread must have used release semantics and the destroying
71 // thread must have used acquire semantics for that communication,
72 // so we are guaranteed to see the post-wait() value of state_,
73 // which cannot be WAITING.
75 // Note that since we only care about a single memory location,
76 // the only two plausible memory orders here are relaxed and seq_cst.
77 assert(state_.load(std::memory_order_relaxed) != WAITING);
80 FOLLY_ALWAYS_INLINE bool ready() const noexcept {
81 auto s = state_.load(std::memory_order_acquire);
82 assert(s == INIT || s == EARLY_DELIVERY);
83 return LIKELY(s == EARLY_DELIVERY);
86 /// Equivalent to destroying the Baton and creating a new one. It is
87 /// a bug to call this while there is a waiting thread, so in practice
88 /// the waiter will be the one that resets the baton.
89 void reset() noexcept {
90 // See ~Baton for a discussion about why relaxed is okay here
91 assert(state_.load(std::memory_order_relaxed) != WAITING);
93 // We use a similar argument to justify the use of a relaxed store
94 // here. Since both wait() and post() are required to be called
95 // only once per lifetime, no thread can actually call those methods
96 // correctly after a reset() unless it synchronizes with the thread
97 // that performed the reset(). If a post() or wait() on another thread
98 // didn't synchronize, then regardless of what operation we performed
99 // here there would be a race on proper use of the Baton's spec
100 // (although not on any particular load and store). Put another way,
101 // we don't need to synchronize here because anybody that might rely
102 // on such synchronization is required by the baton rules to perform
103 // an additional synchronization that has the desired effect anyway.
105 // There is actually a similar argument to be made about the
106 // constructor, in which the fenceless constructor initialization
107 // of state_ is piggybacked on whatever synchronization mechanism
108 // distributes knowledge of the Baton's existence
109 state_.store(INIT, std::memory_order_relaxed);
112 /// Causes wait() to wake up. For each lifetime of a Baton (where a
113 /// lifetime starts at construction or reset() and ends at
114 /// destruction or reset()) there can be at most one call to post(),
115 /// in the single poster version. Any thread may call post().
116 void post() noexcept {
118 /// Spin-only version
121 auto state = state_.load(std::memory_order_relaxed);
122 return (state == INIT || state == EARLY_DELIVERY);
124 state_.store(EARLY_DELIVERY, std::memory_order_release);
128 /// May-block versions
130 uint32_t before = state_.load(std::memory_order_acquire);
132 assert(before == INIT || before == WAITING || before == TIMED_OUT);
134 if (before == INIT &&
135 state_.compare_exchange_strong(
138 std::memory_order_release,
139 std::memory_order_relaxed)) {
143 assert(before == WAITING || before == TIMED_OUT);
145 if (before == TIMED_OUT) {
149 assert(before == WAITING);
150 state_.store(LATE_DELIVERY, std::memory_order_release);
154 /// Waits until post() has been called in the current Baton lifetime.
155 /// May be called at most once during a Baton lifetime (construction
156 /// |reset until destruction|reset). If post is called before wait in
157 /// the current lifetime then this method returns immediately.
159 /// The restriction that there can be at most one wait() per lifetime
160 /// could be relaxed somewhat without any perf or size regressions,
161 /// but by making this condition very restrictive we can provide better
162 /// checking in debug builds.
163 FOLLY_ALWAYS_INLINE void wait() noexcept {
171 /// Similar to wait, but doesn't block the thread if it hasn't been posted.
173 /// try_wait has the following semantics:
174 /// - It is ok to call try_wait any number times on the same baton until
175 /// try_wait reports that the baton has been posted.
176 /// - It is ok to call timed_wait or wait on the same baton if try_wait
177 /// reports that baton hasn't been posted.
178 /// - If try_wait indicates that the baton has been posted, it is invalid to
179 /// call wait, try_wait or timed_wait on the same baton without resetting
181 /// @return true if baton has been posted, false othewise
182 FOLLY_ALWAYS_INLINE bool try_wait() const noexcept {
186 /// Similar to wait, but with a timeout. The thread is unblocked if the
188 /// Note: Only a single call to wait/try_wait_for/try_wait_until is allowed
189 /// during a baton's life-cycle (from ctor/reset to dtor/reset). In other
190 /// words, after try_wait_for the caller can't invoke
191 /// wait/try_wait/try_wait_for/try_wait_until
192 /// again on the same baton without resetting it.
194 /// @param timeout Time until which the thread can block
195 /// @return true if the baton was posted to before timeout,
197 template <typename Rep, typename Period>
198 FOLLY_ALWAYS_INLINE bool try_wait_for(
199 const std::chrono::duration<Rep, Period>& timeout) noexcept {
201 MayBlock, "Non-blocking Baton does not support try_wait_for.");
207 auto deadline = std::chrono::steady_clock::now() + timeout;
208 return tryWaitUntilSlow(deadline);
211 /// Similar to wait, but with a deadline. The thread is unblocked if the
212 /// deadline expires.
213 /// Note: Only a single call to wait/try_wait_for/try_wait_until is allowed
214 /// during a baton's life-cycle (from ctor/reset to dtor/reset). In other
215 /// words, after try_wait_until the caller can't invoke
216 /// wait/try_wait/try_wait_for/try_wait_until
217 /// again on the same baton without resetting it.
219 /// @param deadline Time until which the thread can block
220 /// @return true if the baton was posted to before deadline,
222 template <typename Clock, typename Duration>
223 FOLLY_ALWAYS_INLINE bool try_wait_until(
224 const std::chrono::time_point<Clock, Duration>& deadline) noexcept {
226 MayBlock, "Non-blocking Baton does not support try_wait_until.");
232 return tryWaitUntilSlow(deadline);
235 /// Alias to try_wait_for. Deprecated.
236 template <typename Rep, typename Period>
237 FOLLY_ALWAYS_INLINE bool timed_wait(
238 const std::chrono::duration<Rep, Period>& timeout) noexcept {
239 return try_wait_for(timeout);
242 /// Alias to try_wait_until. Deprecated.
243 template <typename Clock, typename Duration>
244 FOLLY_ALWAYS_INLINE bool timed_wait(
245 const std::chrono::time_point<Clock, Duration>& deadline) noexcept {
246 return try_wait_until(deadline);
250 enum State : uint32_t {
259 // Must be positive. If multiple threads are actively using a
260 // higher-level data structure that uses batons internally, it is
261 // likely that the post() and wait() calls happen almost at the same
262 // time. In this state, we lose big 50% of the time if the wait goes
263 // to sleep immediately. On circa-2013 devbox hardware it costs about
264 // 7 usec to FUTEX_WAIT and then be awoken (half the t/iter as the
265 // posix_sem_pingpong test in BatonTests). We can improve our chances
266 // of EARLY_DELIVERY by spinning for a bit, although we have to balance
267 // this against the loss if we end up sleeping any way. Spins on this
268 // hw take about 7 nanos (all but 0.5 nanos is the pause instruction).
269 // We give ourself 300 spins, which is about 2 usec of waiting. As a
270 // partial consolation, since we are using the pause instruction we
271 // are giving a speed boost to the colocated hyperthread.
272 PreBlockAttempts = 300,
275 // Spin for "some time" (see discussion on PreBlockAttempts) waiting
278 // @return true if we received an early delivery during the wait,
279 // false otherwise. If the function returns true then
280 // state_ is guaranteed to be EARLY_DELIVERY
281 bool spinWaitForEarlyDelivery() noexcept {
283 PreBlockAttempts > 0,
284 "isn't this assert clearer than an uninitialized variable warning?");
285 for (int i = 0; i < PreBlockAttempts; ++i) {
290 // The pause instruction is the polite way to spin, but it doesn't
291 // actually affect correctness to omit it if we don't have it.
292 // Pausing donates the full capabilities of the current core to
293 // its other hyperthreads for a dozen cycles or so
294 asm_volatile_pause();
300 FOLLY_NOINLINE void waitSlow() noexcept {
301 if (spinWaitForEarlyDelivery()) {
302 assert(state_.load(std::memory_order_acquire) == EARLY_DELIVERY);
307 while (!try_wait()) {
308 std::this_thread::yield();
313 // guess we have to block :(
314 uint32_t expected = INIT;
315 if (!state_.compare_exchange_strong(expected, WAITING)) {
316 // CAS failed, last minute reprieve
317 assert(expected == EARLY_DELIVERY);
322 detail::MemoryIdler::futexWait(state_, WAITING);
324 // state_ is the truth even if FUTEX_WAIT reported a matching
325 // FUTEX_WAKE, since we aren't using type-stable storage and we
326 // don't guarantee reuse. The scenario goes like this: thread
327 // A's last touch of a Baton is a call to wake(), which stores
328 // LATE_DELIVERY and gets an unlucky context switch before delivering
329 // the corresponding futexWake. Thread B sees LATE_DELIVERY
330 // without consuming a futex event, because it calls futexWait
331 // with an expected value of WAITING and hence doesn't go to sleep.
332 // B returns, so the Baton's memory is reused and becomes another
333 // Baton (or a reuse of this one). B calls futexWait on the new
334 // Baton lifetime, then A wakes up and delivers a spurious futexWake
335 // to the same memory location. B's futexWait will then report a
336 // consumed wake event even though state_ is still WAITING.
338 // It would be possible to add an extra state_ dance to communicate
339 // that the futexWake has been sent so that we can be sure to consume
340 // it before returning, but that would be a perf and complexity hit.
341 uint32_t s = state_.load(std::memory_order_acquire);
342 assert(s == WAITING || s == LATE_DELIVERY);
344 if (s == LATE_DELIVERY) {
351 template <typename Clock, typename Duration>
352 FOLLY_NOINLINE bool tryWaitUntilSlow(
353 const std::chrono::time_point<Clock, Duration>& deadline) noexcept {
354 if (spinWaitForEarlyDelivery()) {
355 assert(state_.load(std::memory_order_acquire) == EARLY_DELIVERY);
359 // guess we have to block :(
360 uint32_t expected = INIT;
361 if (!state_.compare_exchange_strong(expected, WAITING)) {
362 // CAS failed, last minute reprieve
363 assert(expected == EARLY_DELIVERY);
368 auto rv = state_.futexWaitUntil(WAITING, deadline);
369 if (rv == folly::detail::FutexResult::TIMEDOUT) {
370 state_.store(TIMED_OUT, std::memory_order_release);
374 uint32_t s = state_.load(std::memory_order_acquire);
375 assert(s == WAITING || s == LATE_DELIVERY);
376 if (s == LATE_DELIVERY) {
382 detail::Futex<Atom> state_;