sgx_tstd/src/sync/barrier.rs

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License..

use core::fmt;
use crate::sync::{SgxCondvar, SgxMutex};

/// A barrier enables multiple threads to synchronize the beginning
/// of some computation.
///
pub struct Barrier {
    lock: SgxMutex<BarrierState>,
    cvar: SgxCondvar,
    num_threads: usize,
}

// The inner state of a double barrier
struct BarrierState {
    count: usize,
    generation_id: usize,
}

/// A `BarrierWaitResult` is returned by [`wait`] when all threads in the [`Barrier`]
/// have rendezvoused.
///
/// [`wait`]: struct.Barrier.html#method.wait
/// [`Barrier`]: struct.Barrier.html
///
pub struct BarrierWaitResult(bool);

impl fmt::Debug for Barrier {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("Barrier { .. }")
    }
}

impl Barrier {
    /// Creates a new barrier that can block a given number of threads.
    ///
    /// A barrier will block `n`-1 threads which call [`wait`] and then wake up
    /// all threads at once when the `n`th thread calls [`wait`].
    ///
    /// [`wait`]: #method.wait
    ///
    pub fn new(n: usize) -> Barrier {
        Barrier {
            lock: SgxMutex::new(BarrierState { count: 0, generation_id: 0 }),
            cvar: SgxCondvar::new(),
            num_threads: n,
        }
    }

    /// Blocks the current thread until all threads have rendezvoused here.
    ///
    /// Barriers are re-usable after all threads have rendezvoused once, and can
    /// be used continuously.
    ///
    /// A single (arbitrary) thread will receive a [`BarrierWaitResult`] that
    /// returns `true` from [`is_leader`] when returning from this function, and
    /// all other threads will receive a result that will return `false` from
    /// [`is_leader`].
    ///
    /// [`BarrierWaitResult`]: struct.BarrierWaitResult.html
    /// [`is_leader`]: struct.BarrierWaitResult.html#method.is_leader
    ///
    pub fn wait(&self) -> BarrierWaitResult {
        let mut lock = self.lock.lock().unwrap();
        let local_gen = lock.generation_id;
        lock.count += 1;
        if lock.count < self.num_threads {
            // We need a while loop to guard against spurious wakeups.
            // http://en.wikipedia.org/wiki/Spurious_wakeup
            while local_gen == lock.generation_id && lock.count < self.num_threads {
                lock = self.cvar.wait(lock).unwrap();
            }
            BarrierWaitResult(false)
        } else {
            lock.count = 0;
            lock.generation_id = lock.generation_id.wrapping_add(1);
            self.cvar.notify_all();
            BarrierWaitResult(true)
        }
    }
}

impl fmt::Debug for BarrierWaitResult {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("BarrierWaitResult").field("is_leader", &self.is_leader()).finish()
    }
}

impl BarrierWaitResult {
    /// Returns `true` if this thread from [`wait`] is the "leader thread".
    ///
    /// Only one thread will have `true` returned from their result, all other
    /// threads will have `false` returned.
    ///
    /// [`wait`]: struct.Barrier.html#method.wait
    ///
    pub fn is_leader(&self) -> bool {
        self.0
    }
}