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Add support for acquiring multiple locks at once (#958)
* Add ASLocking which supports -tryLock and taking multiple locks safely * Better multi locking * Assert about lock set capacity
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| Original file line number | Diff line number | Diff line change |
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| // | ||
| // ASLocking.h | ||
| // Texture | ||
| // | ||
| // Copyright (c) 2018-present, Pinterest, Inc. All rights reserved. | ||
| // Licensed 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 | ||
| // | ||
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| #import <Foundation/Foundation.h> | ||
| #import <pthread/sched.h> | ||
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| #import <AsyncDisplayKit/ASAssert.h> | ||
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| NS_ASSUME_NONNULL_BEGIN | ||
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| #define kLockSetCapacity 32 | ||
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| /** | ||
| * An extension of NSLocking that supports -tryLock. | ||
| */ | ||
| @protocol ASLocking <NSLocking> | ||
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| /// Try to take lock without blocking. Returns whether the lock was taken. | ||
| - (BOOL)tryLock; | ||
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| @end | ||
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| /** | ||
| * A set of locks acquired during ASLockSequence. | ||
| */ | ||
| typedef struct { | ||
| unsigned count; | ||
| CFTypeRef _Nullable locks[kLockSetCapacity]; | ||
| } ASLockSet; | ||
|
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| /** | ||
| * Declare a lock set that is automatically unlocked at the end of scope. | ||
| * | ||
| * We use this instead of a scope-locking macro because we want to be able | ||
| * to step through the lock sequence block in the debugger. | ||
| */ | ||
| #define ASScopedLockSet __unused ASLockSet __attribute__((cleanup(ASUnlockSet))) | ||
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| /** | ||
| * A block that attempts to add a lock to a lock sequence. | ||
| * Such a block is provided to the caller of ASLockSequence. | ||
| * | ||
| * Returns whether the lock was added. You should return | ||
| * NO from your lock sequence body if it returns NO. | ||
| * | ||
| * For instance, you might write `return addLock(l1) && addLock(l2)`. | ||
| * | ||
| * @param lock The lock to attempt to add. | ||
| * @return YES if the lock was added, NO otherwise. | ||
| */ | ||
| typedef BOOL(^ASAddLockBlock)(id<ASLocking> lock); | ||
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| /** | ||
| * A block that attempts to lock multiple locks in sequence. | ||
| * Such a block is provided by the caller of ASLockSequence. | ||
| * | ||
| * The block may be run multiple times, if not all locks are immediately | ||
| * available. Therefore the block should be idempotent. | ||
| * | ||
| * The block should attempt to invoke addLock multiple times with | ||
| * different locks. It should return NO as soon as any addLock | ||
| * operation fails. | ||
| * | ||
| * For instance, you might write `return addLock(l1) && addLock(l2)`. | ||
| * | ||
| * @param addLock A block you can call to attempt to add a lock. | ||
| * @return YES if all locks were added, NO otherwise. | ||
| */ | ||
| typedef BOOL(^ASLockSequenceBlock)(NS_NOESCAPE ASAddLockBlock addLock); | ||
|
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| /** | ||
| * Unlock and release all of the locks in this lock set. | ||
| */ | ||
| NS_INLINE void ASUnlockSet(ASLockSet *lockSet) { | ||
| for (unsigned i = 0; i < lockSet->count; i++) { | ||
| CFTypeRef lock = lockSet->locks[i]; | ||
| [(__bridge id<ASLocking>)lock unlock]; | ||
| CFRelease(lock); | ||
| } | ||
| } | ||
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| /** | ||
| * Take multiple locks "simultaneously," avoiding deadlocks | ||
| * caused by lock ordering. | ||
| * | ||
| * The block you provide should attempt to take a series of locks, | ||
| * using the provided `addLock` block. As soon as any addLock fails, | ||
| * you should return NO. | ||
| * | ||
| * For example: | ||
| * ASLockSequence(^(ASAddLockBlock addLock) ^{ | ||
| * return addLock(l0) && addLock(l1); | ||
| * }); | ||
| * | ||
| * Note: This function doesn't protect from lock ordering deadlocks if | ||
| * one of the locks is already locked (recursive.) Only locks taken | ||
| * inside this function are guaranteed not to cause a deadlock. | ||
| */ | ||
| NS_INLINE ASLockSet ASLockSequence(NS_NOESCAPE ASLockSequenceBlock body) | ||
| { | ||
| __block ASLockSet locks = (ASLockSet){0}; | ||
| BOOL (^addLock)(id<ASLocking>) = ^(id<ASLocking> obj) { | ||
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| // nil lock = ignore. | ||
| if (!obj) { | ||
| return YES; | ||
| } | ||
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| // If they go over capacity, assert and return YES. | ||
| // If we return NO, they will enter an infinite loop. | ||
| if (locks.count == kLockSetCapacity) { | ||
| ASDisplayNodeCFailAssert(@"Locking more than %d locks at once is not supported.", kLockSetCapacity); | ||
| return YES; | ||
| } | ||
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| if ([obj tryLock]) { | ||
| locks.locks[locks.count++] = (__bridge_retained CFTypeRef)obj; | ||
| return YES; | ||
| } | ||
| return NO; | ||
| }; | ||
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| /** | ||
| * Repeatedly try running their block, passing in our `addLock` | ||
| * until it succeeds. If it fails, unlock all and yield the thread | ||
| * to reduce spinning. | ||
| */ | ||
| while (true) { | ||
| if (body(addLock)) { | ||
| // Success | ||
| return locks; | ||
| } else { | ||
| ASUnlockSet(&locks); | ||
| locks.count = 0; | ||
| sched_yield(); | ||
| } | ||
| } | ||
| } | ||
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| /** | ||
| * These Foundation classes already implement -tryLock. | ||
| */ | ||
|
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| @interface NSLock (ASLocking) <ASLocking> | ||
| @end | ||
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| @interface NSRecursiveLock (ASLocking) <ASLocking> | ||
| @end | ||
|
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| @interface NSConditionLock (ASLocking) <ASLocking> | ||
| @end | ||
|
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| NS_ASSUME_NONNULL_END |
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