解决死锁问题-open call

本文探讨了在多线程环境下,如何通过开放调用技术避免因资源依赖形成锁环路导致的死锁问题。通过对Taxi调度系统的案例分析,介绍了在调用外部接口时不持有锁的方法,并讨论了这种方法可能牺牲的原子性及其补偿方案。

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当调用一个外部的接口时,如果持有锁,由于不知道外部接口做了什么操作,此时,可能由于资源依赖形成锁环路,造成死锁。解决办法是,使用开放调用,调用外部接口时,不持有锁。

先看看错误的调用:如果有两个线程同时调用Taxi的setLocation和Dispatcher的printLocation,则可能因为错误的执行时序造成两个线程都只获得了一个锁,等待另一个永远无法获得的锁,从而造成死锁。

public class Taxi {

    private Point location, destination;

    private final Dispatcher dispatcher;

    public Taxi(Dispatcher dispatcher) {
        this.dispatcher = dispatcher;
    }

    public synchronized Point getLocation() {
        return location;
    }

    public synchronized void setLocation(Point location) {
        this.location = location;
        if(location.equals(destination)) {
            dispatcher.notifyAvailable(this);
        }
    }

}
public class Dispatcher {

    private final Set<Taxi> taxis;
    private final Set<Taxi> availableTaxis;

    public Dispatcher() {
        taxis = new HashSet<>();
        availableTaxis = new HashSet<>();
    }

    public synchronized void notifyAvailable(Taxi taxi) {
        availableTaxis.add(taxi);
    }

    public synchronized void printLocation() {
        for(Taxi taxi :taxis) {
            System.out.println(taxi.getLocation());
        }
    }
}

开放调用:在调用某个方法时不需要持有锁。

修改Taxi类

public void setLocation(Point location) {
    this.location = location;
    boolean reachedDestination;
    synchronized (this) {
        this.location = location;
        reachedDestination = location.equals(destination);
    }
    if(reachedDestination) {
        dispatcher.notifyAvailable(this);
    }
}

修改Dispatcher类:

public void printLocation() {
    Set<Taxi> copy;
    synchronized (this) {
        copy = new HashSet<>(taxis);
    }
    for(Taxi taxi :taxis) {
        System.out.println(taxi.getLocation());
    }
}

但是,使用开放调用技术改造接口会失去原子性,在大多数情况下,非原子性是可以接受的。如果必须保证原子性,则需要设计一些协议,而不是使用加锁,来补偿原子性。例如,使用单线程执行所有任务。

USE [DXQcontrol_L1HIMCP1_Operational_W] GO /****** Object: StoredProcedure [dbo].[prodExecmgmtIdent__processBR] Script Date: 7/29/2025 9:03:21 AM ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO /*----------------------------------------------------------------------- Project: Duerr EcoEmos - Module Ident Stored Procedure: dbo.prodExecmgmtIdent__processBR Author: JM Initial date: 11. Nov 2020 Revisions: rev 1 - 11. Nov 2020, JM: wrapper function for the old OrderManagement to call dbo.prodExecmgmtIdent__processDR Input: @arg_dateEvt PLC timestamp of the reading event @arg_szRWStationTag Tagname for the reader (configured in table DS_CfgRegistrationPoint_TAB and for FB369 in addition in file IdentServer.ini) @arg_szDatasetIn Tracking data from PLC Output: @arg_szDatasetOut The mds data string which was merged by the new data from PLC and the last stored data string from DS_MDS_TAB Return: See the return codes of the several variables with name @iRetCode... ------------------------------------------------------------------------*/ CREATE PROCEDURE [dbo].[prodExecmgmtIdent__processBR] @arg_dateEvt DATETIME ,@arg_szRWStationTag NStringMin ,@arg_szDatasetIn VARCHAR(MAX) ,@arg_szDatasetOut VARCHAR(MAX) OUTPUT AS BEGIN SET NOCOUNT ON; SET XACT_ABORT ON; SET DEADLOCK_PRIORITY HIGH; DECLARE @object_name VARCHAR(128) = OBJECT_SCHEMA_NAME(@@PROCID) + '.' + OBJECT_NAME(@@PROCID); ---------------------------------------------------------------------------------------------------- -- PRREQ-1908: avoid start of process with an open or uncommitted transaction DECLARE @log VARCHAR(MAX) = ''; DECLARE @tempTable TABLE(logExt VARCHAR(MAX)); IF XACT_STATE() <> 0 BEGIN SET @log = 'Start ' + @object_name + '; '; SET @log += '@@TRANCOUNT = ' + CAST(@@TRANCOUNT AS VARCHAR(MAX)) + '; '; SET @log += 'XACT_STATE() = ' + CAST(XACT_STATE() AS VARCHAR(MAX)) + '; '; SET @log += '@@spid = ' + CAST(@@spid AS VARCHAR(MAX)) + '; '; SET @log += 'ProcParam: '; SET @log += 'rwStation: ' + @arg_szRWStationTag + '; '; SET @log += 'bodyId: ' + dbo.DS_FCT_GetPLCValueOf_(@arg_szDatasetIn, 'bodyId') + '; '; SET @log += 'skidId: ' + dbo.DS_FCT_GetPLCValueOf_(@arg_szDatasetIn, 'skidId') + '; '; SET @log += 'skidType: ' + dbo.DS_FCT_GetPLCValueOf_(@arg_szDatasetIn, 'skidType') + '; '; INSERT INTO @tempTable (logExt) SELECT DISTINCT 'spid=' + CAST(S.spid AS VARCHAR(MAX)) + '; hostname=' + RTRIM(S.hostname) + '; program_name=' + RTRIM(S.program_name) + '; hostprocess=' + RTRIM(S.hostprocess) + '; text=' + D.text + '; lastwaittype=' + RTRIM(S.lastwaittype) FROM sys.sysprocesses S CROSS APPLY sys.dm_exec_sql_text(S.sql_handle) D WHERE S.open_tran > 0; --ORDER BY S.hostname, S.program_name; ROLLBACK TRANSACTION; INSERT INTO dbo.DS_MDSErrorLog_TAB (dateEvt, rwStationId, source, errorCode, errorText) SELECT GETDATE(), @arg_szRWStationTag, @object_name, 'WARNING', isnull(logExt,'-') FROM @tempTable; EXEC dbo.DS_SP_MDSWriteToErrorLog @arg_rwStationId = @arg_szRWStationTag, @arg_szSource = @object_name, @arg_szErrorCode = 'WARNING', @arg_szErrorText = @log; END; ---------------------------------------------------------------------------------------------------- DECLARE @iRetValue INT; ------------------------------------------------------------------------------- -- write this information into log EXEC [dbo].[DS_SP_MDSWriteToLogger] @arg_szRWStationTag, @object_name, @arg_szDatasetIn; ------------------------------------------------------------------------------- -- set the prober function code SET @arg_szDatasetIn = STUFF( @arg_szDatasetIn, 1, 2, 'DR'); EXEC @iRetValue = [dbo].[prodExecmgmtIdent__processDR] @arg_dateEvt, -- DATETIME @arg_szRWStationTag, -- NStringMin @arg_szDatasetIn, -- VARCHAR(MAX) @arg_szDatasetOut OUTPUT; -- VARCHAR(MAX) RETURN @iRetValue; END; GO 详细解析此sqlserver的功能
最新发布
07-30
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