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Change sync::RWlock implementation to use atomic uint instead of exclusive, for performance. Close #7066.

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This commit is contained in:
Ben Blum 2013-06-10 22:13:56 -04:00
parent 68e8fe9b6e
commit 57cb44dbeb
1 changed files with 80 additions and 57 deletions
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137
src/libextra/sync.rs
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@ -20,7 +20,8 @@ use core::prelude::*;
use core::borrow;
use core::comm;
use core::task;
use core::unstable::sync::{Exclusive, exclusive};
use core::unstable::sync::{Exclusive, exclusive, UnsafeAtomicRcBox};
use core::unstable::atomics;
use core::util;
/****************************************************************************
@ -37,6 +38,7 @@ type SignalEnd = comm::ChanOne<()>;
struct Waitqueue { head: comm::Port<SignalEnd>,
tail: comm::Chan<SignalEnd> }
#[doc(hidden)]
fn new_waitqueue() -> Waitqueue {
let (block_head, block_tail) = comm::stream();
Waitqueue { head: block_head, tail: block_tail }
@ -166,9 +168,12 @@ impl Sem<~[Waitqueue]> {
// FIXME(#3588) should go inside of access()
#[doc(hidden)]
type SemRelease<'self> = SemReleaseGeneric<'self, ()>;
#[doc(hidden)]
type SemAndSignalRelease<'self> = SemReleaseGeneric<'self, ~[Waitqueue]>;
#[doc(hidden)]
struct SemReleaseGeneric<'self, Q> { sem: &'self Sem<Q> }
#[doc(hidden)]
#[unsafe_destructor]
impl<'self, Q:Owned> Drop for SemReleaseGeneric<'self, Q> {
fn finalize(&self) {
@ -176,12 +181,14 @@ impl<'self, Q:Owned> Drop for SemReleaseGeneric<'self, Q> {
}
}
#[doc(hidden)]
fn SemRelease<'r>(sem: &'r Sem<()>) -> SemRelease<'r> {
SemReleaseGeneric {
sem: sem
}
}
#[doc(hidden)]
fn SemAndSignalRelease<'r>(sem: &'r Sem<~[Waitqueue]>)
-> SemAndSignalRelease<'r> {
SemReleaseGeneric {
@ -465,8 +472,23 @@ impl Mutex {
#[doc(hidden)]
struct RWlockInner {
// You might ask, "Why don't you need to use an atomic for the mode flag?"
// This flag affects the behaviour of readers (for plain readers, they
// assert on it; for downgraders, they use it to decide which mode to
// unlock for). Consider that the flag is only unset when the very last
// reader exits; therefore, it can never be unset during a reader/reader
// (or reader/downgrader) race.
// By the way, if we didn't care about the assert in the read unlock path,
// we could instead store the mode flag in write_downgrade's stack frame,
// and have the downgrade tokens store a borrowed pointer to it.
read_mode: bool,
read_count: uint
// The only way the count flag is ever accessed is with xadd. Since it is
// a read-modify-write operation, multiple xadds on different cores will
// always be consistent with respect to each other, so a monotonic/relaxed
// consistency ordering suffices (i.e., no extra barriers are needed).
// FIXME(#6598): The atomics module has no relaxed ordering flag, so I use
// acquire/release orderings superfluously. Change these someday.
read_count: atomics::AtomicUint,
}
/**
@ -479,7 +501,7 @@ struct RWlockInner {
pub struct RWlock {
priv order_lock: Semaphore,
priv access_lock: Sem<~[Waitqueue]>,
priv state: Exclusive<RWlockInner>
priv state: UnsafeAtomicRcBox<RWlockInner>,
}
/// Create a new rwlock, with one associated condvar.
@ -490,10 +512,13 @@ pub fn RWlock() -> RWlock { rwlock_with_condvars(1) }
* Similar to mutex_with_condvars.
*/
pub fn rwlock_with_condvars(num_condvars: uint) -> RWlock {
RWlock { order_lock: semaphore(1),
let state = UnsafeAtomicRcBox::new(RWlockInner {
read_mode: false,
read_count: atomics::AtomicUint::new(0),
});
RWlock { order_lock: semaphore(1),
access_lock: new_sem_and_signal(1, num_condvars),
state: exclusive(RWlockInner { read_mode: false,
read_count: 0 }) }
state: state, }
}
impl RWlock {
@ -513,20 +538,11 @@ impl RWlock {
unsafe {
do task::unkillable {
do (&self.order_lock).access {
let mut first_reader = false;
do self.state.with |state| {
first_reader = (state.read_count == 0);
state.read_count += 1;
}
if first_reader {
let state = &mut *self.state.get();
let old_count = state.read_count.fetch_add(1, atomics::Acquire);
if old_count == 0 {
(&self.access_lock).acquire();
do self.state.with |state| {
// Must happen *after* getting access_lock. If
// this is set while readers are waiting, but
// while a writer holds the lock, the writer will
// be confused if they downgrade-then-unlock.
state.read_mode = true;
}
state.read_mode = true;
}
}
release = Some(RWlockReleaseRead(self));
@ -606,12 +622,12 @@ impl RWlock {
* # Example
*
* ~~~ {.rust}
* do lock.write_downgrade |write_mode| {
* do (&write_mode).write_cond |condvar| {
* do lock.write_downgrade |write_token| {
* do (&write_token).write_cond |condvar| {
* ... exclusive access ...
* }
* let read_mode = lock.downgrade(write_mode);
* do (&read_mode).read {
* let read_token = lock.downgrade(write_token);
* do (&read_token).read {
* ... shared access ...
* }
* }
@ -640,14 +656,15 @@ impl RWlock {
}
unsafe {
do task::unkillable {
let mut first_reader = false;
do self.state.with |state| {
assert!(!state.read_mode);
state.read_mode = true;
first_reader = (state.read_count == 0);
state.read_count += 1;
}
if !first_reader {
let state = &mut *self.state.get();
assert!(!state.read_mode);
state.read_mode = true;
// If a reader attempts to enter at this point, both the
// downgrader and reader will set the mode flag. This is fine.
let old_count = state.read_count.fetch_add(1, atomics::Release);
// If another reader was already blocking, we need to hand-off
// the "reader cloud" access lock to them.
if old_count != 0 {
// Guaranteed not to let another writer in, because
// another reader was holding the order_lock. Hence they
// must be the one to get the access_lock (because all
@ -667,22 +684,22 @@ struct RWlockReleaseRead<'self> {
lock: &'self RWlock,
}
#[doc(hidden)]
#[unsafe_destructor]
impl<'self> Drop for RWlockReleaseRead<'self> {
fn finalize(&self) {
unsafe {
do task::unkillable {
let mut last_reader = false;
do self.lock.state.with |state| {
assert!(state.read_mode);
assert!(state.read_count > 0);
state.read_count -= 1;
if state.read_count == 0 {
last_reader = true;
state.read_mode = false;
}
}
if last_reader {
let state = &mut *self.lock.state.get();
assert!(state.read_mode);
let old_count = state.read_count.fetch_sub(1, atomics::Release);
assert!(old_count > 0);
if old_count == 1 {
state.read_mode = false;
// Note: this release used to be outside of a locked access
// to exclusive-protected state. If this code is ever
// converted back to such (instead of using atomic ops),
// this access MUST NOT go inside the exclusive access.
(&self.lock.access_lock).release();
}
}
@ -690,6 +707,7 @@ impl<'self> Drop for RWlockReleaseRead<'self> {
}
}
#[doc(hidden)]
fn RWlockReleaseRead<'r>(lock: &'r RWlock) -> RWlockReleaseRead<'r> {
RWlockReleaseRead {
lock: lock
@ -703,30 +721,34 @@ struct RWlockReleaseDowngrade<'self> {
lock: &'self RWlock,
}
#[doc(hidden)]
#[unsafe_destructor]
impl<'self> Drop for RWlockReleaseDowngrade<'self> {
fn finalize(&self) {
unsafe {
do task::unkillable {
let mut writer_or_last_reader = false;
do self.lock.state.with |state| {
if state.read_mode {
assert!(state.read_count > 0);
state.read_count -= 1;
if state.read_count == 0 {
// Case 1: Writer downgraded & was the last reader
writer_or_last_reader = true;
state.read_mode = false;
} else {
// Case 2: Writer downgraded & was not the last
// reader
}
} else {
// Case 3: Writer did not downgrade
let writer_or_last_reader;
// Check if we're releasing from read mode or from write mode.
let state = &mut *self.lock.state.get();
if state.read_mode {
// Releasing from read mode.
let old_count = state.read_count.fetch_sub(1, atomics::Release);
assert!(old_count > 0);
// Check if other readers remain.
if old_count == 1 {
// Case 1: Writer downgraded & was the last reader
writer_or_last_reader = true;
state.read_mode = false;
} else {
// Case 2: Writer downgraded & was not the last reader
writer_or_last_reader = false;
}
} else {
// Case 3: Writer did not downgrade
writer_or_last_reader = true;
}
if writer_or_last_reader {
// Nobody left inside; release the "reader cloud" lock.
(&self.lock.access_lock).release();
}
}
@ -734,6 +756,7 @@ impl<'self> Drop for RWlockReleaseDowngrade<'self> {
}
}
#[doc(hidden)]
fn RWlockReleaseDowngrade<'r>(lock: &'r RWlock)
-> RWlockReleaseDowngrade<'r> {
RWlockReleaseDowngrade {