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//! Lock types that observe lock acquisition order.
//!
//! This module's [`Mutex`] type is instrumented to observe the
//! nesting of `wgpu-core` lock acquisitions. Whenever `wgpu-core`
//! acquires one lock while it is already holding another, we note
//! that nesting pair. This tells us what the [`LockRank::followers`]
//! set for each lock would need to include to accommodate
//! `wgpu-core`'s observed behavior.
//!
//! When `wgpu-core`'s `observe_locks` feature is enabled, if the
//! `WGPU_CORE_LOCK_OBSERVE_DIR` environment variable is set to the
//! path of an existing directory, then every thread that acquires a
//! lock in `wgpu-core` will write its own log file to that directory.
//! You can then run the `wgpu` workspace's `lock-analyzer` binary to
//! read those files and summarize the results. The output from
//! `lock-analyzer` has the same form as the lock ranks given in
//! [`lock/rank.rs`].
//!
//! If the `WGPU_CORE_LOCK_OBSERVE_DIR` environment variable is not
//! set, then no instrumentation takes place, and the locks behave
//! normally.
//!
//! To make sure we capture all acquisitions regardless of when the
//! program exits, each thread writes events directly to its log file
//! as they occur. A `write` system call is generally just a copy from
//! userspace into the kernel's buffer, so hopefully this approach
//! will still have tolerable performance.
//!
//! [`lock/rank.rs`]: ../../../src/wgpu_core/lock/rank.rs.html
use crate::FastHashSet;
use super::rank::{LockRank, LockRankSet};
use std::{
cell::RefCell,
fs::File,
panic::Location,
path::{Path, PathBuf},
};
/// A `Mutex` instrumented for lock acquisition order observation.
///
/// This is just a wrapper around a [`parking_lot::Mutex`], along with
/// its rank in the `wgpu_core` lock ordering.
///
/// For details, see [the module documentation][self].
pub struct Mutex<T> {
inner: parking_lot::Mutex<T>,
rank: LockRank,
}
/// A guard produced by locking [`Mutex`].
///
/// This is just a wrapper around a [`parking_lot::MutexGuard`], along
/// with the state needed to track lock acquisition.
///
/// For details, see [the module documentation][self].
pub struct MutexGuard<'a, T> {
inner: parking_lot::MutexGuard<'a, T>,
_state: LockStateGuard,
}
impl<T> Mutex<T> {
pub fn new(rank: LockRank, value: T) -> Mutex<T> {
Mutex {
inner: parking_lot::Mutex::new(value),
rank,
}
}
#[track_caller]
pub fn lock(&self) -> MutexGuard<T> {
let saved = acquire(self.rank, Location::caller());
MutexGuard {
inner: self.inner.lock(),
_state: LockStateGuard { saved },
}
}
}
impl<'a, T> std::ops::Deref for MutexGuard<'a, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.inner.deref()
}
}
impl<'a, T> std::ops::DerefMut for MutexGuard<'a, T> {
fn deref_mut(&mut self) -> &mut Self::Target {
self.inner.deref_mut()
}
}
impl<T: std::fmt::Debug> std::fmt::Debug for Mutex<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
self.inner.fmt(f)
}
}
/// An `RwLock` instrumented for lock acquisition order observation.
///
/// This is just a wrapper around a [`parking_lot::RwLock`], along with
/// its rank in the `wgpu_core` lock ordering.
///
/// For details, see [the module documentation][self].
pub struct RwLock<T> {
inner: parking_lot::RwLock<T>,
rank: LockRank,
}
/// A read guard produced by locking [`RwLock`] for reading.
///
/// This is just a wrapper around a [`parking_lot::RwLockReadGuard`], along with
/// the state needed to track lock acquisition.
///
/// For details, see [the module documentation][self].
pub struct RwLockReadGuard<'a, T> {
inner: parking_lot::RwLockReadGuard<'a, T>,
_state: LockStateGuard,
}
/// A write guard produced by locking [`RwLock`] for writing.
///
/// This is just a wrapper around a [`parking_lot::RwLockWriteGuard`], along
/// with the state needed to track lock acquisition.
///
/// For details, see [the module documentation][self].
pub struct RwLockWriteGuard<'a, T> {
inner: parking_lot::RwLockWriteGuard<'a, T>,
_state: LockStateGuard,
}
impl<T> RwLock<T> {
pub fn new(rank: LockRank, value: T) -> RwLock<T> {
RwLock {
inner: parking_lot::RwLock::new(value),
rank,
}
}
#[track_caller]
pub fn read(&self) -> RwLockReadGuard<T> {
let saved = acquire(self.rank, Location::caller());
RwLockReadGuard {
inner: self.inner.read(),
_state: LockStateGuard { saved },
}
}
#[track_caller]
pub fn write(&self) -> RwLockWriteGuard<T> {
let saved = acquire(self.rank, Location::caller());
RwLockWriteGuard {
inner: self.inner.write(),
_state: LockStateGuard { saved },
}
}
}
impl<'a, T> RwLockWriteGuard<'a, T> {
pub fn downgrade(this: Self) -> RwLockReadGuard<'a, T> {
RwLockReadGuard {
inner: parking_lot::RwLockWriteGuard::downgrade(this.inner),
_state: this._state,
}
}
}
impl<T: std::fmt::Debug> std::fmt::Debug for RwLock<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
self.inner.fmt(f)
}
}
impl<'a, T> std::ops::Deref for RwLockReadGuard<'a, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.inner.deref()
}
}
impl<'a, T> std::ops::Deref for RwLockWriteGuard<'a, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.inner.deref()
}
}
impl<'a, T> std::ops::DerefMut for RwLockWriteGuard<'a, T> {
fn deref_mut(&mut self) -> &mut Self::Target {
self.inner.deref_mut()
}
}
/// A container that restores a prior per-thread lock state when dropped.
///
/// This type serves two purposes:
///
/// - Operations like `RwLockWriteGuard::downgrade` would like to be able to
/// destructure lock guards and reassemble their pieces into new guards, but
/// if the guard type itself implements `Drop`, we can't destructure it
/// without unsafe code or pointless `Option`s whose state is almost always
/// statically known.
///
/// - We can just implement `Drop` for this type once, and then use it in lock
/// guards, rather than implementing `Drop` separately for each guard type.
struct LockStateGuard {
/// The youngest lock that was already held when we acquired this
/// one, if any.
saved: Option<HeldLock>,
}
impl Drop for LockStateGuard {
fn drop(&mut self) {
release(self.saved)
}
}
/// Check and record the acquisition of a lock with `new_rank`.
///
/// Log the acquisition of a lock with `new_rank`, and
/// update the per-thread state accordingly.
///
/// Return the `Option<HeldLock>` state that must be restored when this lock is
/// released.
fn acquire(new_rank: LockRank, location: &'static Location<'static>) -> Option<HeldLock> {
LOCK_STATE.with_borrow_mut(|state| match *state {
ThreadState::Disabled => None,
ThreadState::Initial => {
let Ok(dir) = std::env::var("WGPU_CORE_LOCK_OBSERVE_DIR") else {
*state = ThreadState::Disabled;
return None;
};
// Create the observation log file.
let mut log = ObservationLog::create(dir)
.expect("Failed to open lock observation file (does the dir exist?)");
// Log the full set of lock ranks, so that the analysis can even see
// locks that are only acquired in isolation.
for rank in LockRankSet::all().iter() {
log.write_rank(rank);
}
// Update our state to reflect that we are logging acquisitions, and
// that we have acquired this lock.
*state = ThreadState::Enabled {
held_lock: Some(HeldLock {
rank: new_rank,
location,
}),
log,
};
// Since this is the first acquisition on this thread, we know that
// there is no prior lock held, and thus nothing to log yet.
None
}
ThreadState::Enabled {
ref mut held_lock,
ref mut log,
} => {
if let Some(ref held_lock) = held_lock {
log.write_acquisition(held_lock, new_rank, location);
}
std::mem::replace(
held_lock,
Some(HeldLock {
rank: new_rank,
location,
}),
)
}
})
}
/// Record the release of a lock whose saved state was `saved`.
fn release(saved: Option<HeldLock>) {
LOCK_STATE.with_borrow_mut(|state| {
if let ThreadState::Enabled {
ref mut held_lock, ..
} = *state
{
*held_lock = saved;
}
});
}
thread_local! {
static LOCK_STATE: RefCell<ThreadState> = const { RefCell::new(ThreadState::Initial) };
}
/// Thread-local state for lock observation.
enum ThreadState {
/// This thread hasn't yet checked the environment variable.
Initial,
/// This thread checked the environment variable, and it was
/// unset, so this thread is not observing lock acquisitions.
Disabled,
/// Lock observation is enabled for this thread.
Enabled {
held_lock: Option<HeldLock>,
log: ObservationLog,
},
}
/// Information about a currently held lock.
#[derive(Debug, Copy, Clone)]
struct HeldLock {
/// The lock's rank.
rank: LockRank,
/// Where we acquired the lock.
location: &'static Location<'static>,
}
/// A log to which we can write observations of lock activity.
struct ObservationLog {
/// The file to which we are logging lock observations.
log_file: File,
/// [`Location`]s we've seen so far.
///
/// This is a hashset of raw pointers because raw pointers have
/// the [`Eq`] and [`Hash`] relations we want: the pointer value, not
/// the contents. There's no unsafe code in this module.
locations_seen: FastHashSet<*const Location<'static>>,
/// Buffer for serializing events, retained for allocation reuse.
buffer: Vec<u8>,
}
#[allow(trivial_casts)]
impl ObservationLog {
/// Create an observation log in `dir` for the current pid and thread.
fn create(dir: impl AsRef<Path>) -> Result<Self, std::io::Error> {
let mut path = PathBuf::from(dir.as_ref());
path.push(format!(
"locks-{}.{:?}.ron",
std::process::id(),
std::thread::current().id()
));
let log_file = File::create(&path)?;
Ok(ObservationLog {
log_file,
locations_seen: FastHashSet::default(),
buffer: Vec::new(),
})
}
/// Record the acquisition of one lock while holding another.
///
/// Log that we acquired a lock of `new_rank` at `new_location` while still
/// holding other locks, the most recently acquired of which has
/// `older_rank`.
fn write_acquisition(
&mut self,
older_lock: &HeldLock,
new_rank: LockRank,
new_location: &'static Location<'static>,
) {
self.write_location(older_lock.location);
self.write_location(new_location);
self.write_action(&Action::Acquisition {
older_rank: older_lock.rank.bit.number(),
older_location: addr(older_lock.location),
newer_rank: new_rank.bit.number(),
newer_location: addr(new_location),
});
}
fn write_location(&mut self, location: &'static Location<'static>) {
if self.locations_seen.insert(location) {
self.write_action(&Action::Location {
address: addr(location),
file: location.file(),
line: location.line(),
column: location.column(),
});
}
}
fn write_rank(&mut self, rank: LockRankSet) {
self.write_action(&Action::Rank {
bit: rank.number(),
member_name: rank.member_name(),
const_name: rank.const_name(),
});
}
fn write_action(&mut self, action: &Action) {
use std::io::Write;
self.buffer.clear();
ron::ser::to_writer(&mut self.buffer, &action)
.expect("error serializing `lock::observing::Action`");
self.buffer.push(b'\n');
self.log_file
.write_all(&self.buffer)
.expect("error writing `lock::observing::Action`");
}
}
/// An action logged by a thread that is observing lock acquisition order.
///
/// Each thread's log file is a sequence of these enums, serialized
/// using the [`ron`] crate, one action per line.
///
/// Lock observation cannot assume that there will be any convenient
/// finalization point before the program exits, so in practice,
/// actions must be written immediately when they occur. This means we
/// can't, say, accumulate tables and write them out when they're
/// complete. The `lock-analyzer` binary is then responsible for
/// consolidating the data into a single table of observed transitions.
#[derive(serde::Serialize)]
enum Action {
/// A location that we will refer to in later actions.
///
/// We write one of these events the first time we see a
/// particular `Location`. Treating this as a separate action
/// simply lets us avoid repeating the content over and over
/// again in every [`Acquisition`] action.
///
/// [`Acquisition`]: Action::Acquisition
Location {
address: usize,
file: &'static str,
line: u32,
column: u32,
},
/// A lock rank that we will refer to in later actions.
///
/// We write out one these events for every lock rank at the
/// beginning of each thread's log file. Treating this as a
/// separate action simply lets us avoid repeating the names over
/// and over again in every [`Acquisition`] action.
///
/// [`Acquisition`]: Action::Acquisition
Rank {
bit: u32,
member_name: &'static str,
const_name: &'static str,
},
/// An attempt to acquire a lock while holding another lock.
Acquisition {
/// The number of the already acquired lock's rank.
older_rank: u32,
/// The source position at which we acquired it. Specifically,
/// its `Location`'s address, as an integer.
older_location: usize,
/// The number of the rank of the lock we are acquiring.
newer_rank: u32,
/// The source position at which we are acquiring it.
/// Specifically, its `Location`'s address, as an integer.
newer_location: usize,
},
}
impl LockRankSet {
/// Return the number of this rank's first member.
fn number(self) -> u32 {
self.bits().trailing_zeros()
}
}
/// Convenience for `std::ptr::from_ref(t) as usize`.
fn addr<T>(t: &T) -> usize {
std::ptr::from_ref(t) as usize
}