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This commit is contained in:
John Kåre Alsaker 2018-05-27 13:05:41 +02:00
parent 302aae5864
commit c819ba043a
1 changed files with 92 additions and 27 deletions
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119
src/librustc/ty/maps/job.rs
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@ -194,19 +194,25 @@ impl<'tcx> QueryLatch<'tcx> {
}
}
/// Awaits the caller on this latch by blocking the current thread.
fn await(&self, waiter: &mut QueryWaiter<'tcx>) {
let mut info = self.info.lock();
if !info.complete {
// We push the waiter on to the `waiters` list. It can be accessed inside
// the `wait` call below, by 1) the `set` method or 2) by deadlock detection.
// Both of these will remove it from the `waiters` list before resuming
// this thread.
info.waiters.push(waiter);
let condvar = &waiter.condvar;
// If this detects a deadlock and the deadlock handler want to resume this thread
// we have to be in the `wait` call. This is ensured by the deadlock handler
// getting the self.info lock.
rayon_core::mark_blocked();
condvar.wait(&mut info);
waiter.condvar.wait(&mut info);
}
}
/// Sets the latch and resumes all waiters on it
fn set(&self) {
let mut info = self.info.lock();
debug_assert!(!info.complete);
@ -219,46 +225,56 @@ impl<'tcx> QueryLatch<'tcx> {
}
}
fn resume_waiter(
/// Remove a single waiter from the list of waiters.
/// This is used to break query cycles.
fn extract_waiter(
&self,
waiter: usize,
error: CycleError<'tcx>
) -> *mut QueryWaiter<'tcx> {
let mut info = self.info.lock();
debug_assert!(!info.complete);
// Remove the waiter from the list of waiters
let waiter = info.waiters.remove(waiter);
// Set the cycle error it will be picked it up when resumed
unsafe {
(*waiter).cycle = Some(error);
}
waiter
info.waiters.remove(waiter)
}
}
/// A pointer to an active query job. This is used to give query jobs an identity.
#[cfg(parallel_queries)]
type Ref<'tcx> = *const QueryJob<'tcx>;
/// A resumable waiter of a query. The usize is the index into waiters in the query's latch
#[cfg(parallel_queries)]
type Waiter<'tcx> = (Ref<'tcx>, usize);
/// Visits all the non-resumable and resumable waiters of a query.
/// Only waiters in a query are visited.
/// `visit` is called for every waiter and is passed a query waiting on `query_ref`
/// and a span indicating the reason the query waited on `query_ref`.
/// If `visit` returns Some, this function returns.
/// For visits of non-resumable waiters it returns the return value of `visit`.
/// For visits of resumable waiters it returns Some(Some(Waiter)) which has the
/// required information to resume the waiter.
/// If all `visit` calls returns None, this function also returns None.
#[cfg(parallel_queries)]
fn visit_waiters<'tcx, F>(query_ref: Ref<'tcx>, mut visit: F) -> Option<Option<Waiter<'tcx>>>
where
F: FnMut(Span, Ref<'tcx>) -> Option<Option<Waiter<'tcx>>>
{
let query = unsafe { &*query_ref };
// Visit the parent query which is a non-resumable waiter since it's on the same stack
if let Some(ref parent) = query.parent {
if let Some(cycle) = visit(query.info.span, &**parent as Ref) {
return Some(cycle);
}
}
// Visit the explict waiters which use condvars and are resumable
for (i, &waiter) in query.latch.info.lock().waiters.iter().enumerate() {
unsafe {
if let Some(ref waiter_query) = *(*waiter).query {
if visit((*waiter).span, &**waiter_query as Ref).is_some() {
// Return a value which indicates that this waiter can be resumed
return Some(Some((query_ref, i)));
}
}
@ -267,6 +283,10 @@ where
None
}
/// Look for query cycles by doing a depth first search starting at `query`.
/// `span` is the reason for the `query` to execute. This is initially DUMMY_SP.
/// If a cycle is detected, this initial value is replaced with the span causing
/// the cycle.
#[cfg(parallel_queries)]
fn cycle_check<'tcx>(query: Ref<'tcx>,
span: Span,
@ -274,6 +294,8 @@ fn cycle_check<'tcx>(query: Ref<'tcx>,
visited: &mut HashSet<Ref<'tcx>>) -> Option<Option<Waiter<'tcx>>> {
if visited.contains(&query) {
return if let Some(p) = stack.iter().position(|q| q.1 == query) {
// We detected a query cycle, fix up the initial span and return Some
// Remove previous stack entries
stack.splice(0..p, iter::empty());
// Replace the span for the first query with the cycle cause
@ -284,13 +306,16 @@ fn cycle_check<'tcx>(query: Ref<'tcx>,
}
}
// Mark this query is visited and add it to the stack
visited.insert(query);
stack.push((span, query));
// Visit all the waiters
let r = visit_waiters(query, |span, successor| {
cycle_check(successor, span, stack, visited)
});
// Remove the entry in our stack if we didn't find a cycle
if r.is_none() {
stack.pop();
}
@ -298,12 +323,17 @@ fn cycle_check<'tcx>(query: Ref<'tcx>,
r
}
/// Finds out if there's a path to the compiler root (aka. code which isn't in a query)
/// from `query` without going through any of the queries in `visited`.
/// This is achieved with a depth first search.
#[cfg(parallel_queries)]
fn connected_to_root<'tcx>(query: Ref<'tcx>, visited: &mut HashSet<Ref<'tcx>>) -> bool {
// We already visited this or we're deliberately ignoring it
if visited.contains(&query) {
return false;
}
// This query is connected to the root (it has no query parent), return true
if unsafe { (*query).parent.is_none() } {
return true;
}
@ -321,19 +351,20 @@ fn connected_to_root<'tcx>(query: Ref<'tcx>, visited: &mut HashSet<Ref<'tcx>>) -
}).is_some()
}
#[cfg(parallel_queries)]
fn query_entry<'tcx>(r: Ref<'tcx>) -> QueryInfo<'tcx> {
unsafe { (*r).info.clone() }
}
/// Looks for query cycles starting from the last query in `jobs`.
/// If a cycle is found, all queries in the cycle is removed from `jobs` and
/// the function return true.
/// If a cycle was not found, the starting query is removed from `jobs` and
/// the function returns false.
#[cfg(parallel_queries)]
fn remove_cycle<'tcx>(
jobs: &mut Vec<Ref<'tcx>>,
wakelist: &mut Vec<*mut QueryWaiter<'tcx>>,
tcx: TyCtxt<'_, 'tcx, '_>
) {
) -> bool {
let mut visited = HashSet::new();
let mut stack = Vec::new();
// Look for a cycle starting with the last query in `jobs`
if let Some(waiter) = cycle_check(jobs.pop().unwrap(),
DUMMY_SP,
&mut stack,
@ -341,13 +372,15 @@ fn remove_cycle<'tcx>(
// Reverse the stack so earlier entries require later entries
stack.reverse();
// Extract the spans and queries into separate arrays
let mut spans: Vec<_> = stack.iter().map(|e| e.0).collect();
let queries = stack.iter().map(|e| e.1);
// Shift the spans so that a query is matched the span for its waitee
// Shift the spans so that queries are matched with the span for their waitee
let last = spans.pop().unwrap();
spans.insert(0, last);
// Zip them back together
let mut stack: Vec<_> = spans.into_iter().zip(queries).collect();
// Remove the queries in our cycle from the list of jobs to look at
@ -355,9 +388,6 @@ fn remove_cycle<'tcx>(
jobs.remove_item(&r.1);
}
let (waitee_query, waiter_idx) = waiter.unwrap();
let waitee_query = unsafe { &*waitee_query };
// Find the queries in the cycle which are
// connected to queries outside the cycle
let entry_points: Vec<Ref<'_>> = stack.iter().filter_map(|query| {
@ -392,6 +422,7 @@ fn remove_cycle<'tcx>(
stack.insert(0, last);
}
// Create the cycle error
let mut error = CycleError {
usage: None,
cycle: stack.iter().map(|&(s, q)| QueryInfo {
@ -400,10 +431,30 @@ fn remove_cycle<'tcx>(
} ).collect(),
};
wakelist.push(waitee_query.latch.resume_waiter(waiter_idx, error));
// We unwrap `waiter` here since there must always be one
// edge which is resumeable / waited using a query latch
let (waitee_query, waiter_idx) = waiter.unwrap();
let waitee_query = unsafe { &*waitee_query };
// Extract the waiter we want to resume
let waiter = waitee_query.latch.extract_waiter(waiter_idx);
// Set the cycle error it will be picked it up when resumed
unsafe {
(*waiter).cycle = Some(error);
}
// Put the waiter on the list of things to resume
wakelist.push(waiter);
true
} else {
false
}
}
/// Creates a new thread and forwards information in thread locals to it.
/// The new thread runs the deadlock handler.
#[cfg(parallel_queries)]
pub fn handle_deadlock() {
use syntax;
@ -440,6 +491,11 @@ pub fn handle_deadlock() {
});
}
/// Detects query cycles by using depth first search over all active query jobs.
/// If a query cycle is found it will break the cycle by finding an edge which
/// uses a query latch and then resuming that waiter.
/// There may be multiple cycles involved in a deadlock, so this searches
/// all active queries for cycles before finally resuming all the waiters at once.
#[cfg(parallel_queries)]
fn deadlock(tcx: TyCtxt<'_, '_, '_>, registry: &rayon_core::Registry) {
let on_panic = OnDrop(|| {
@ -450,13 +506,22 @@ fn deadlock(tcx: TyCtxt<'_, '_, '_>, registry: &rayon_core::Registry) {
let mut wakelist = Vec::new();
let mut jobs: Vec<_> = tcx.maps.collect_active_jobs().iter().map(|j| &**j as Ref).collect();
let mut found_cycle = false;
while jobs.len() > 0 {
remove_cycle(&mut jobs, &mut wakelist, tcx);
if remove_cycle(&mut jobs, &mut wakelist, tcx) {
found_cycle = true;
}
}
// FIXME: Panic if no cycle is detected
// FIXME: Write down the conditions when a deadlock happens without a cycle
// Check that a cycle was found. It is possible for a deadlock to occur without
// a query cycle if a query which can be waited on uses Rayon to do multithreading
// internally. Such a query (X) may be executing on 2 threads (A and B) and A may
// wait using Rayon on B. Rayon may then switch to executing another query (Y)
// which in turn will wait on X causing a deadlock. We have a false dependency from
// X to Y due to Rayon waiting and a true dependency from Y to X. The algorithm here
// only considers the true dependency and won't detect a cycle.
assert!(found_cycle);
// FIXME: Ensure this won't cause a deadlock before we return
for waiter in wakelist.into_iter() {