From c5afd254bd0147de752d72de40548842cf5afeed Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Thu, 29 Aug 2019 18:53:36 -0700
Subject: [PATCH] lockmanager: sort key by bucket in
 QuorumLockManager::doLockByType

Change-Id: Ida9b3979a7d6ad36e669761f2e23eeef6c67efae
---
 .../libs/lockmanager/QuorumLockManager.php    | 22 +++++++++++--------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/includes/libs/lockmanager/QuorumLockManager.php b/includes/libs/lockmanager/QuorumLockManager.php
index 950b283670..6478a61b14 100644
--- a/includes/libs/lockmanager/QuorumLockManager.php
+++ b/includes/libs/lockmanager/QuorumLockManager.php
@@ -38,7 +38,7 @@ abstract class QuorumLockManager extends LockManager {
 	final protected function doLockByType( array $pathsByType ) {
 		$status = StatusValue::newGood();
 
-		$pathsToLock = []; // (bucket => type => paths)
+		$pathsByTypeByBucket = []; // (bucket => type => paths)
 		// Get locks that need to be acquired (buckets => locks)...
 		foreach ( $pathsByType as $type => $paths ) {
 			foreach ( $paths as $path ) {
@@ -46,23 +46,27 @@ abstract class QuorumLockManager extends LockManager {
 					++$this->locksHeld[$path][$type];
 				} else {
 					$bucket = $this->getBucketFromPath( $path );
-					$pathsToLock[$bucket][$type][] = $path;
+					$pathsByTypeByBucket[$bucket][$type][] = $path;
 				}
 			}
 		}
 
+		// Acquire locks in each bucket in bucket order to reduce contention. Any blocking
+		// mutexes during the acquisition step will not involve circular waiting on buckets.
+		ksort( $pathsByTypeByBucket );
+
 		$lockedPaths = []; // files locked in this attempt (type => paths)
 		// Attempt to acquire these locks...
-		foreach ( $pathsToLock as $bucket => $pathsToLockByType ) {
+		foreach ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
 			// Try to acquire the locks for this bucket
-			$status->merge( $this->doLockingRequestBucket( $bucket, $pathsToLockByType ) );
+			$status->merge( $this->doLockingRequestBucket( $bucket, $bucketPathsByType ) );
 			if ( !$status->isOK() ) {
 				$status->merge( $this->doUnlockByType( $lockedPaths ) );
 
 				return $status;
 			}
 			// Record these locks as active
-			foreach ( $pathsToLockByType as $type => $paths ) {
+			foreach ( $bucketPathsByType as $type => $paths ) {
 				foreach ( $paths as $path ) {
 					$this->locksHeld[$path][$type] = 1; // locked
 					// Keep track of what locks were made in this attempt
@@ -77,7 +81,7 @@ abstract class QuorumLockManager extends LockManager {
 	protected function doUnlockByType( array $pathsByType ) {
 		$status = StatusValue::newGood();
 
-		$pathsToUnlock = []; // (bucket => type => paths)
+		$pathsByTypeByBucket = []; // (bucket => type => paths)
 		foreach ( $pathsByType as $type => $paths ) {
 			foreach ( $paths as $path ) {
 				if ( !isset( $this->locksHeld[$path][$type] ) ) {
@@ -88,7 +92,7 @@ abstract class QuorumLockManager extends LockManager {
 					if ( $this->locksHeld[$path][$type] <= 0 ) {
 						unset( $this->locksHeld[$path][$type] );
 						$bucket = $this->getBucketFromPath( $path );
-						$pathsToUnlock[$bucket][$type][] = $path;
+						$pathsByTypeByBucket[$bucket][$type][] = $path;
 					}
 					if ( $this->locksHeld[$path] === [] ) {
 						unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
@@ -99,8 +103,8 @@ abstract class QuorumLockManager extends LockManager {
 
 		// Remove these specific locks if possible, or at least release
 		// all locks once this process is currently not holding any locks.
-		foreach ( $pathsToUnlock as $bucket => $pathsToUnlockByType ) {
-			$status->merge( $this->doUnlockingRequestBucket( $bucket, $pathsToUnlockByType ) );
+		foreach ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
+			$status->merge( $this->doUnlockingRequestBucket( $bucket, $bucketPathsByType ) );
 		}
 		if ( $this->locksHeld === [] ) {
 			$status->merge( $this->releaseAllLocks() );
-- 
2.20.1