1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 /// This is the flavor of channels which are not necessarily optimized for any
14 /// particular use case, but are the most general in how they are used. Shared
15 /// channels are cloneable allowing for multiple senders.
17 /// High level implementation details can be found in the comment of the parent
18 /// module. You'll also note that the implementation of the shared and stream
19 /// channels are quite similar, and this is no coincidence!
21 pub use self::Failure::*;
25 use alloc::boxed::Box;
28 use rustrt::local::Local;
29 use rustrt::task::{Task, BlockedTask};
30 use rustrt::thread::Thread;
32 use sync::{atomic, Mutex, MutexGuard};
33 use comm::mpsc_queue as mpsc;
35 const DISCONNECTED: int = int::MIN;
36 const FUDGE: int = 1024;
38 const MAX_STEALS: int = 5;
40 const MAX_STEALS: int = 1 << 20;
42 pub struct Packet<T> {
43 queue: mpsc::Queue<T>,
44 cnt: atomic::AtomicInt, // How many items are on this channel
45 steals: int, // How many times has a port received without blocking?
46 to_wake: atomic::AtomicUint, // Task to wake up
48 // The number of channels which are currently using this packet.
49 channels: atomic::AtomicInt,
51 // See the discussion in Port::drop and the channel send methods for what
53 port_dropped: atomic::AtomicBool,
54 sender_drain: atomic::AtomicInt,
56 // this lock protects various portions of this implementation during
58 select_lock: Mutex<()>,
66 impl<T: Send> Packet<T> {
67 // Creation of a packet *must* be followed by a call to postinit_lock
68 // and later by inherit_blocker
69 pub fn new() -> Packet<T> {
71 queue: mpsc::Queue::new(),
72 cnt: atomic::AtomicInt::new(0),
74 to_wake: atomic::AtomicUint::new(0),
75 channels: atomic::AtomicInt::new(2),
76 port_dropped: atomic::AtomicBool::new(false),
77 sender_drain: atomic::AtomicInt::new(0),
78 select_lock: Mutex::new(()),
83 // This function should be used after newly created Packet
84 // was wrapped with an Arc
85 // In other case mutex data will be duplicated while cloning
86 // and that could cause problems on platforms where it is
87 // represented by opaque data structure
88 pub fn postinit_lock(&self) -> MutexGuard<()> {
89 self.select_lock.lock()
92 // This function is used at the creation of a shared packet to inherit a
93 // previously blocked task. This is done to prevent spurious wakeups of
96 // This can only be called at channel-creation time
97 pub fn inherit_blocker(&mut self,
98 task: Option<BlockedTask>,
99 guard: MutexGuard<()>) {
102 assert_eq!(self.cnt.load(atomic::SeqCst), 0);
103 assert_eq!(self.to_wake.load(atomic::SeqCst), 0);
104 self.to_wake.store(unsafe { task.cast_to_uint() },
106 self.cnt.store(-1, atomic::SeqCst);
108 // This store is a little sketchy. What's happening here is
109 // that we're transferring a blocker from a oneshot or stream
110 // channel to this shared channel. In doing so, we never
111 // spuriously wake them up and rather only wake them up at the
112 // appropriate time. This implementation of shared channels
113 // assumes that any blocking recv() will undo the increment of
114 // steals performed in try_recv() once the recv is complete.
115 // This thread that we're inheriting, however, is not in the
116 // middle of recv. Hence, the first time we wake them up,
117 // they're going to wake up from their old port, move on to the
118 // upgraded port, and then call the block recv() function.
120 // When calling this function, they'll find there's data
121 // immediately available, counting it as a steal. This in fact
122 // wasn't a steal because we appropriately blocked them waiting
125 // To offset this bad increment, we initially set the steal
126 // count to -1. You'll find some special code in
127 // abort_selection() as well to ensure that this -1 steal count
128 // doesn't escape too far.
134 // When the shared packet is constructed, we grabbed this lock. The
135 // purpose of this lock is to ensure that abort_selection() doesn't
136 // interfere with this method. After we unlock this lock, we're
137 // signifying that we're done modifying self.cnt and self.to_wake and
138 // the port is ready for the world to continue using it.
142 pub fn send(&mut self, t: T) -> Result<(), T> {
143 // See Port::drop for what's going on
144 if self.port_dropped.load(atomic::SeqCst) { return Err(t) }
146 // Note that the multiple sender case is a little trickier
147 // semantically than the single sender case. The logic for
148 // incrementing is "add and if disconnected store disconnected".
149 // This could end up leading some senders to believe that there
150 // wasn't a disconnect if in fact there was a disconnect. This means
151 // that while one thread is attempting to re-store the disconnected
152 // states, other threads could walk through merrily incrementing
153 // this very-negative disconnected count. To prevent senders from
154 // spuriously attempting to send when the channels is actually
155 // disconnected, the count has a ranged check here.
157 // This is also done for another reason. Remember that the return
158 // value of this function is:
160 // `true` == the data *may* be received, this essentially has no
162 // `false` == the data will *never* be received, this has a lot of
165 // In the SPSC case, we have a check of 'queue.is_empty()' to see
166 // whether the data was actually received, but this same condition
167 // means nothing in a multi-producer context. As a result, this
168 // preflight check serves as the definitive "this will never be
169 // received". Once we get beyond this check, we have permanently
170 // entered the realm of "this may be received"
171 if self.cnt.load(atomic::SeqCst) < DISCONNECTED + FUDGE {
176 match self.cnt.fetch_add(1, atomic::SeqCst) {
178 self.take_to_wake().wake().map(|t| t.reawaken());
181 // In this case, we have possibly failed to send our data, and
182 // we need to consider re-popping the data in order to fully
183 // destroy it. We must arbitrate among the multiple senders,
184 // however, because the queues that we're using are
185 // single-consumer queues. In order to do this, all exiting
186 // pushers will use an atomic count in order to count those
187 // flowing through. Pushers who see 0 are required to drain as
188 // much as possible, and then can only exit when they are the
189 // only pusher (otherwise they must try again).
190 n if n < DISCONNECTED + FUDGE => {
191 // see the comment in 'try' for a shared channel for why this
192 // window of "not disconnected" is ok.
193 self.cnt.store(DISCONNECTED, atomic::SeqCst);
195 if self.sender_drain.fetch_add(1, atomic::SeqCst) == 0 {
197 // drain the queue, for info on the thread yield see the
198 // discussion in try_recv
200 match self.queue.pop() {
202 mpsc::Empty => break,
203 mpsc::Inconsistent => Thread::yield_now(),
206 // maybe we're done, if we're not the last ones
207 // here, then we need to go try again.
208 if self.sender_drain.fetch_sub(1, atomic::SeqCst) == 1 {
213 // At this point, there may still be data on the queue,
214 // but only if the count hasn't been incremented and
215 // some other sender hasn't finished pushing data just
216 // yet. That sender in question will drain its own data.
220 // Can't make any assumptions about this case like in the SPSC case.
227 pub fn recv(&mut self) -> Result<T, Failure> {
228 // This code is essentially the exact same as that found in the stream
229 // case (see stream.rs)
230 match self.try_recv() {
235 let task: Box<Task> = Local::take();
236 task.deschedule(1, |task| {
240 match self.try_recv() {
241 data @ Ok(..) => { self.steals -= 1; data }
246 // Essentially the exact same thing as the stream decrement function.
247 fn decrement(&mut self, task: BlockedTask) -> Result<(), BlockedTask> {
248 assert_eq!(self.to_wake.load(atomic::SeqCst), 0);
249 let n = unsafe { task.cast_to_uint() };
250 self.to_wake.store(n, atomic::SeqCst);
252 let steals = self.steals;
255 match self.cnt.fetch_sub(1 + steals, atomic::SeqCst) {
256 DISCONNECTED => { self.cnt.store(DISCONNECTED, atomic::SeqCst); }
257 // If we factor in our steals and notice that the channel has no
258 // data, we successfully sleep
261 if n - steals <= 0 { return Ok(()) }
265 self.to_wake.store(0, atomic::SeqCst);
266 Err(unsafe { BlockedTask::cast_from_uint(n) })
269 pub fn try_recv(&mut self) -> Result<T, Failure> {
270 let ret = match self.queue.pop() {
271 mpsc::Data(t) => Some(t),
274 // This is a bit of an interesting case. The channel is
275 // reported as having data available, but our pop() has
276 // failed due to the queue being in an inconsistent state.
277 // This means that there is some pusher somewhere which has
278 // yet to complete, but we are guaranteed that a pop will
279 // eventually succeed. In this case, we spin in a yield loop
280 // because the remote sender should finish their enqueue
281 // operation "very quickly".
283 // Avoiding this yield loop would require a different queue
284 // abstraction which provides the guarantee that after M
285 // pushes have succeeded, at least M pops will succeed. The
286 // current queues guarantee that if there are N active
287 // pushes, you can pop N times once all N have finished.
288 mpsc::Inconsistent => {
292 match self.queue.pop() {
293 mpsc::Data(t) => { data = t; break }
294 mpsc::Empty => panic!("inconsistent => empty"),
295 mpsc::Inconsistent => {}
302 // See the discussion in the stream implementation for why we
303 // might decrement steals.
305 if self.steals > MAX_STEALS {
306 match self.cnt.swap(0, atomic::SeqCst) {
308 self.cnt.store(DISCONNECTED, atomic::SeqCst);
311 let m = cmp::min(n, self.steals);
316 assert!(self.steals >= 0);
322 // See the discussion in the stream implementation for why we try
325 match self.cnt.load(atomic::SeqCst) {
326 n if n != DISCONNECTED => Err(Empty),
328 match self.queue.pop() {
329 mpsc::Data(t) => Ok(t),
330 mpsc::Empty => Err(Disconnected),
331 // with no senders, an inconsistency is impossible.
332 mpsc::Inconsistent => unreachable!(),
340 // Prepares this shared packet for a channel clone, essentially just bumping
342 pub fn clone_chan(&mut self) {
343 self.channels.fetch_add(1, atomic::SeqCst);
346 // Decrement the reference count on a channel. This is called whenever a
347 // Chan is dropped and may end up waking up a receiver. It's the receiver's
348 // responsibility on the other end to figure out that we've disconnected.
349 pub fn drop_chan(&mut self) {
350 match self.channels.fetch_sub(1, atomic::SeqCst) {
352 n if n > 1 => return,
353 n => panic!("bad number of channels left {}", n),
356 match self.cnt.swap(DISCONNECTED, atomic::SeqCst) {
357 -1 => { self.take_to_wake().wake().map(|t| t.reawaken()); }
359 n => { assert!(n >= 0); }
363 // See the long discussion inside of stream.rs for why the queue is drained,
364 // and why it is done in this fashion.
365 pub fn drop_port(&mut self) {
366 self.port_dropped.store(true, atomic::SeqCst);
367 let mut steals = self.steals;
369 let cnt = self.cnt.compare_and_swap(
370 steals, DISCONNECTED, atomic::SeqCst);
371 cnt != DISCONNECTED && cnt != steals
373 // See the discussion in 'try_recv' for why we yield
374 // control of this thread.
376 match self.queue.pop() {
377 mpsc::Data(..) => { steals += 1; }
378 mpsc::Empty | mpsc::Inconsistent => break,
384 // Consumes ownership of the 'to_wake' field.
385 fn take_to_wake(&mut self) -> BlockedTask {
386 let task = self.to_wake.load(atomic::SeqCst);
387 self.to_wake.store(0, atomic::SeqCst);
389 unsafe { BlockedTask::cast_from_uint(task) }
392 ////////////////////////////////////////////////////////////////////////////
393 // select implementation
394 ////////////////////////////////////////////////////////////////////////////
396 // Helper function for select, tests whether this port can receive without
397 // blocking (obviously not an atomic decision).
399 // This is different than the stream version because there's no need to peek
400 // at the queue, we can just look at the local count.
401 pub fn can_recv(&mut self) -> bool {
402 let cnt = self.cnt.load(atomic::SeqCst);
403 cnt == DISCONNECTED || cnt - self.steals > 0
406 // increment the count on the channel (used for selection)
407 fn bump(&mut self, amt: int) -> int {
408 match self.cnt.fetch_add(amt, atomic::SeqCst) {
410 self.cnt.store(DISCONNECTED, atomic::SeqCst);
417 // Inserts the blocked task for selection on this port, returning it back if
418 // the port already has data on it.
420 // The code here is the same as in stream.rs, except that it doesn't need to
421 // peek at the channel to see if an upgrade is pending.
422 pub fn start_selection(&mut self,
423 task: BlockedTask) -> Result<(), BlockedTask> {
424 match self.decrement(task) {
427 let prev = self.bump(1);
428 assert!(prev == DISCONNECTED || prev >= 0);
434 // Cancels a previous task waiting on this port, returning whether there's
437 // This is similar to the stream implementation (hence fewer comments), but
438 // uses a different value for the "steals" variable.
439 pub fn abort_selection(&mut self, _was_upgrade: bool) -> bool {
440 // Before we do anything else, we bounce on this lock. The reason for
441 // doing this is to ensure that any upgrade-in-progress is gone and
442 // done with. Without this bounce, we can race with inherit_blocker
443 // about looking at and dealing with to_wake. Once we have acquired the
444 // lock, we are guaranteed that inherit_blocker is done.
446 let _guard = self.select_lock.lock();
449 // Like the stream implementation, we want to make sure that the count
450 // on the channel goes non-negative. We don't know how negative the
451 // stream currently is, so instead of using a steal value of 1, we load
452 // the channel count and figure out what we should do to make it
455 let cnt = self.cnt.load(atomic::SeqCst);
456 if cnt < 0 && cnt != DISCONNECTED {-cnt} else {0}
458 let prev = self.bump(steals + 1);
460 if prev == DISCONNECTED {
461 assert_eq!(self.to_wake.load(atomic::SeqCst), 0);
464 let cur = prev + steals + 1;
467 self.take_to_wake().trash();
469 while self.to_wake.load(atomic::SeqCst) != 0 {
473 // if the number of steals is -1, it was the pre-emptive -1 steal
474 // count from when we inherited a blocker. This is fine because
475 // we're just going to overwrite it with a real value.
476 assert!(self.steals == 0 || self.steals == -1);
477 self.steals = steals;
484 impl<T: Send> Drop for Packet<T> {
486 // Note that this load is not only an assert for correctness about
487 // disconnection, but also a proper fence before the read of
488 // `to_wake`, so this assert cannot be removed with also removing
489 // the `to_wake` assert.
490 assert_eq!(self.cnt.load(atomic::SeqCst), DISCONNECTED);
491 assert_eq!(self.to_wake.load(atomic::SeqCst), 0);
492 assert_eq!(self.channels.load(atomic::SeqCst), 0);