I have inherited some code that has a set of real-time values that are captured over a serial link that runs on a separate thread:
class Data
{
public static int nFooCount;
public static decimal meanValue;
// Lots more of a variety of types.
}
The thread just stores the data into the field with no locking or other synchronization. Looks like a whole sea of race conditions to me. So I want to add some safety to it. The question is, what's best?
I could make the int fields volatile. Can't do that with the decimal types, though. Interlocked can help with that, albeit messily, using boxing. Or I could add a lock object
private static readonly object lockObj = new object();
and then accessor everything. But this locks all the fields even when it will only modify one at a time. I can't lock on primitives and that would be bad form anyway, and adding a lock object for every field would look wasteful. So is there a better way?
You could lock according to required access level using ReaderWriterLockSlim. You could do this better by turning them into a property:
public int Something
{
get {
locker.EnterReadLock();
try {
return something;
} finally {
locker.ExitReadLock();
}
}
set {
locker.EnterWriteLock();
try {
something = value;
} finally {
locker.ExitWriteLock();
}
}
}
This allows for multiple reads and single writes.
Related
Is it possible to easily convert a property in a C# that uses locking in its getter and setter so that it uses a ReaderWriterLock instead?
For instance, consider the following property:
object numLock = new object();
public int Num
{
get
{
Lock(numLock)
{
return num;
}
}
set
{
Lock(numLock)
{
num = value;
}
}
}
What is the fastest way to change this to allow multiple readers or a single writer. Note that the code that need to be changed contain hundreds of similar properties. So, I'm looking for a simple solution to convert the locking mechanism in these properties in a fast and reliable way.
I have a static class 'Logger' with a public property called 'LogLevels' as in code below.
When the property is used concurrently in a multi-user or multi-threaded environment, could it cause problems?
Do I need to use thread synchronization for the code within the property 'LogLevels'?
public class Logger
{
private static List<LogLevel> _logLevels = null;
public static List<LogLevel> LogLevels
{
get
{
if (_logLevels == null)
{
_logLevels = new List<LogLevel>();
if (!string.IsNullOrWhiteSpace(System.Configuration.ConfigurationManager.AppSettings["LogLevels"]))
{
string[] lls = System.Configuration.ConfigurationManager.AppSettings["LogLevels"].Split(",".ToCharArray());
foreach (string ll in lls)
{
_logLevels.Add((LogLevel)System.Enum.Parse(typeof(LogLevel), ll));
}
}
}
if (_logLevels.Count == 0)
{
_logLevels.Add(LogLevel.Error);
}
return _logLevels;
}
}
}
UPDATE: I ended up using thread synchronization to solve concurrency problem in a static class, as in code below.
public class Logger
{
private static readonly System.Object _object = new System.Object();
private static List<LogLevel> _logLevels = null;
private static List<LogLevel> LogLevels
{
get
{
//Make sure that in a multi-threaded or multi-user scenario, we do not run into concurrency issues with this code.
lock (_object)
{
if (_logLevels == null)
{
_logLevels = new List<LogLevel>();
if (!string.IsNullOrWhiteSpace(System.Configuration.ConfigurationManager.AppSettings["SimpleDBLogLevelsLogger"]))
{
string[] lls = System.Configuration.ConfigurationManager.AppSettings["SimpleDBLogLevelsLogger"].Split(",".ToCharArray());
foreach (string ll in lls)
{
_logLevels.Add((LogLevel)System.Enum.Parse(typeof(LogLevel), ll));
}
}
}
if (_logLevels.Count == 0)
{
_logLevels.Add(LogLevel.Error);
}
}
return _logLevels;
}
}
}
When the property is used concurrently in a multi-user or multi-threaded environment, could it cause problems?
Absolutely. List<T> is not designed for multiple threads, except for the case where there are just multiple readers (no writers).
Do I need to use thread synchronization for the code within the property 'LogLevels'?
Well that's one approach. Or just initialize it on type initialization, and then return a read-only wrapper around it. (You really don't want multiple threads modifying it.)
Note that in general, doing significant amounts of work in a static constructor is a bad idea. Are you happy enough that if this fails, every access to this property will fail, forever?
This code posses race conditions and cannot be safely executed from multiple threads. The primary problem is the List<T> type is not thread safe yet this code will freely write to. This mean the writes can occur in parallel and hence break the implicit contract of List<T>
The short answer is "yes" and "yes" you do need threads synchronization.
The other question is, why re-invent the wheel? You can use something like log4net or .NET logging framework.
I would like to have a global object similar to a multi-value Dictionary that is shared among different Threads.
I would like the object to be created only once (for example getting the data from a Database) and then used by the different Threads.
The Object should be easily extendable with additional properties (currently have only JobName and URL).
If possible, I would prefer to avoid locking.
I am facing the following issues:
The current version displayed below is not Thread safe;
I cannot use a ConcurrentDictionary since I have extended the Dictionary object to allow multiple values for each key;
This is the object structure that should be modified easily:
public struct JobData
{
public string JobName;
public string URL;
}
I have extended the Dictionary object to allow multiple values for each key:
public class JobsDictionary : Dictionary<string, JobData>
{
public void Add(string key, string jobName, string url)
{
JobData data;
data.JobName = jobName;
data.URL = url;
this.Add(key, data);
}
}
Static class that is shared among Threads.
As you can see it creates a Dictionary entry for the specific Job the first time it is called for that Job.
For instance, the first time it is called for "earnings" it will create the "earnings" dictionary entry. This creates issues with Thread safety:
public static class GlobalVar
{
private static JobsDictionary jobsDictionary = new JobsDictionary();
public static JobData Job(string jobCat)
{
if (jobsDictionary.ContainsKey(jobCat))
return jobsDictionary[jobCat];
else
{
String jobName;
String url = null;
//TODO: get the Data from the Database
switch (jobCat)
{
case "earnings":
jobName="EarningsWhispers";
url = "http://www.earningswhispers.com/stocks.asp?symbol={0}";
break;
case "stock":
jobName="YahooStock";
url = "http://finance.yahoo.com/q?s={0}";
break;
case "functions":
jobName = "Functions";
url = null;
break;
default:
jobName = null;
url = null;
break;
}
jobsDictionary.Add(jobCat, jobName, url);
return jobsDictionary[jobCat];
}
}
In each Thread I get the specific Job property in this way:
//Get the Name
string JobName= GlobalVar.Job(jobName).JobName;
//Get the URL
string URL = string.Format((GlobalVar.Job(jobName).URL), sym);
How can I create a custom Dictionary that is "instantiated" once (I know it is not the right term since it is static...) and it is Thread-safe ?
Thanks
UPDATE
Ok, here is the new version.
I have simplified the code by removing the switch statement and loading all dictionary items at once (I need all of them anyway).
The advantage of this solution is that it is locked only once: when the dictionary data is added (the first Thread entering the lock will add data to the dictionary).
When the Threads access the dictionary for reading, it is not locked.
It should be Thread-Safe and it should not incur in deadlocks since jobsDictionary is private.
public static class GlobalVar
{
private static JobsDictionary jobsDictionary = new JobsDictionary();
public static JobData Job(string jobCat)
{
JobData result;
if (jobsDictionary.TryGetValue(jobCat, out result))
return result;
//if the jobsDictionary is not initialized yet...
lock (jobsDictionary)
{
if (jobsDictionary.Count == 0)
{
//TODO: get the Data from the Database
jobsDictionary.Add("earnings", "EarningsWhispers", "http://www.earningswhispers.com/stocks.asp?symbol={0}");
jobsDictionary.Add("stock", "YahooStock", "http://finance.yahoo.com/q?s={0}");
jobsDictionary.Add("functions", "Functions", null);
}
return jobsDictionary[jobCat];
}
}
}
If you are populating the collection once, you don't need any locking at all, since a Dictionary is thread-safe when it is only read from. If you want prevent multiple threads from initializing multiple times you can use a double-checked lock during initalization, like this:
static readonly object syncRoot = new object();
static Dictionary<string, JobData> cache;
static void Initialize()
{
if (cache == null)
{
lock (syncRoot)
{
if (cache == null)
{
cache = LoadFromDatabase();
}
}
}
}
Instead of allowing every thread to access the dictionary, hide it behind a facade that only exposes the operations you really need. This makes it much easier to reason about thread-safety. For instance:
public class JobDataCache : IJobData
{
readonly object syncRoot = new object();
Dictionary<string, JobData> cache;
public void AddJob(string key, JobData data)
{
lock (this.syncRoot)
{
cache[key] = data;
}
}
}
Trying to prevent locking without having measured that locking actually has a too big impact on performance is bad. Prevent doing that. Often using a simple lock statement is much simpler than writing lock-free code. There is a nasty problem with concurrency bugs compared to normal software bugs. They are very hard to reproduce and very hard to track down. If you can, prevent writing concurrency bugs. You can do this by writing the simplest code you can, even if it is slower. If it proves to be too slow, you can always optimize.
If you want to write lock-free code anyway, try using immutable data structures, or prevent changing existing data. This is one trick I used when writing the Simple Injector (a reusable library). In this framework, I never update the internal dictionary, but always completely replace it with a new one. The dictionary itself is therefore never changed, the reference to that instance is just replaced with a completely new dictionary. This prevents you from having to do locks completely. However, you must realize that it is possible to loose updates. In other words, when multiple threads are updating that dictionary, one can loose its changes, simply because each thread creates a new copy of that dictionary and adds its own value too its own copy, before making that reference public to other threads.
In other words, you can only use this method when external callers only read (and you can recover from lost changes, for instance by querying the database again).
UPDATE
Your updated version is still not thread-safe, because of the reasons I explained on #ili's answer. The following will do the trick:
public static class GlobalVar
{
private static readonly object syncRoot = new object();
private static JobsDictionary jobsDictionary = null;
public static JobData Job(string jobCat)
{
Initialize();
return jobsDictionary[jobCat];
}
private void Initialize()
{
// Double-checked lock.
if (jobsDictionary == null)
{
lock (syncRoot)
{
if (jobsDictionary == null)
{
jobsDictionary = CreateJobsDictionary();
}
}
}
}
private static JobsDictionary CreateJobsDictionary()
{
var jobs = new JobsDictionary();
//TODO: get the Data from the Database
jobs.Add("earnings", "EarningsWhispers", "http://...");
jobs.Add("stock", "YahooStock", "http://...");
jobs.Add("functions", "Functions", null);
return jobs;
}
}
You can also use the static constructor, which would prevent you from having to write the double checked lock yourself. However, it is dangarous to call the database inside a static constructor, because a static constructor will only run once and when it fails, the complete type will be unusable for as long as the AppDomain lives. In other words your application must be restarted when this happens.
UPDATE 2:
You can also use .NET 4.0's Lazy<T>, which is safer than a double checked lock, since it is easier to implement (and easier to implement correctly) and is is also thread-safe on processor architectures with weak memory models (weaker than x86 such as ARM):
static Lazy<Dictionary<string, JobData>> cache =
new Lazy<Dictionary<string, JobData>>(() => LoadFromDatabase());
1) Use singleton patern to have one instance (one of the ways is to use static class as you have done)
2) To make anything thread safe you should use lock or it's analog. If you are afraids of unnessessary locks do like this:
public object GetValue(object key)
{
object result;
if(_dictionary.TryGetValue(key, out result)
return result;
lock(_dictionary)
{
if(_dictionary.TryGetValue(key, out result)
return result;
//some get data code
_dictionary[key]=result;
return result;
}
}
Say I have a property whose setter is protected by a lock, but without any lock around the getter, e.g.
private long _myField;
public long MyProperty
{
get { return _myField; }
set { lock(whatever) _myField = value; }
}
In addition to synchronizing writes (but not reads), the lock, or rather Monitor.Exit, should cause a volatile write. Let's now say we have two threads A and B, and the following sequence happens:
A reads the current value of MyProperty.
B writes a new value to MyProperty.
A reads the current value of MyProperty again.
Q: Is A now guaranteed to see the new value? Or did our lock just ensure that B writes to main memory in a timely manner, but not that other threads read a fresh value? Or could the answer even depend on whether we're running in .Net 2+ or a "weaker" ECMA implementation?
No, since the read does not have the explicit memory barrier, it is not "guaranteed" to see the new value.
You can use a ReaderWriterLockSlim to insure that a) the writes lock each other and b) the reads always pickup the new value.
private readonly ReaderWriterLockSlim _myFieldLock = new ReaderWriterLockSlim();
private long _myField;
public long MyProperty
{
get
{
_myFieldLock.EnterReadLock();
try
{
return _myField;
}
finally
{
_myFieldLock.ExitReadLock();
}
}
set
{
_myFieldLock.EnterWriteLock();
try
{
_myField = value;
}
finally
{
_myFieldLock.ExitWriteLock();
}
}
}
If you used Interlocked.Read in the getter you should always read the new value. See Threading in C# for more information about memory fences
Often, when I want a class which is thread-safe, I do something like the following:
public class ThreadSafeClass
{
private readonly object theLock = new object();
private double propertyA;
public double PropertyA
{
get
{
lock (theLock)
{
return propertyA;
}
}
set
{
lock (theLock)
{
propertyA = value;
}
}
}
private double propertyB;
public double PropertyB
{
get
{
lock (theLock)
{
return propertyB;
}
}
set
{
lock (theLock)
{
propertyB = value;
}
}
}
public void SomeMethod()
{
lock (theLock)
{
PropertyA = 2.0 * PropertyB;
}
}
}
It works, but it is very verbose. Sometimes I even create a lock object for each method and property creating more verbosity and complexity.
I know that it is also possible to lock classes using the Synchronization attribute but I'm not sure how well that scales -- as I often expect to have hundreds of thousands, if not millions, of instances of thread-safe objects. This approach would create a synchronization context for every instance of the class, and requires the class to be derived from ContextBoundObject and therefore could not be derived from anything else -- since C# doesn't allow for multiple inheritance -- which is a show stopper in many cases.
Edit: As several of the responders have emphasized, there is no "silver bullet" thread-safe class design. I'm just trying to understand if the pattern I'm using is one of the good solutions. Of course the best solution in any particular situation is problem dependent. Several of the answers below contain alternative designs which should be considered.
Edit: Moreover, there is more than one definition of thread safety. For example, in my implementation above, the following code would NOT be thread-safe:
var myObject = new ThreadSafeClass();
myObject.PropertyA++; // NOT thread-safe
So, does the class definition above represent a good approach? If not, what would you recommend for a design with similar behavior which would be thread-safe for a similar set of uses?
There is no "one-size-fits-all" solution to the multi-threading problem. Do some research on creating immutable classes and learn about the different synchronization primitives.
This is an example of a semi-immutable or the-programmers-immutable class .
public class ThreadSafeClass
{
public double A { get; private set; }
public double B { get; private set; }
public double C { get; private set; }
public ThreadSafeClass(double a, double b, double c)
{
A = a;
B = b;
C = c;
}
public ThreadSafeClass RecalculateA()
{
return new ThreadSafeClass(2.0 * B, B, C);
}
}
This example moves your synchronization code into another class and serializes access to an instance. In reality, you don't really want more than one thread operating on an object at any given time.
public class ThreadSafeClass
{
public double PropertyA { get; set; }
public double PropertyB { get; set; }
public double PropertyC { get; set; }
private ThreadSafeClass()
{
}
public void ModifyClass()
{
// do stuff
}
public class Synchronizer
{
private ThreadSafeClass instance = new ThreadSafeClass();
private readonly object locker = new object();
public void Execute(Action<ThreadSafeClass> action)
{
lock (locker)
{
action(instance);
}
}
public T Execute<T>(Func<ThreadSafeClass, T> func)
{
lock (locker)
{
return func(instance);
}
}
}
}
Here is a quick example of how you would use it. It may seem a little clunky but it allows you to execute many actions on the instance in one go.
var syn = new ThreadSafeClass.Synchronizer();
syn.Execute(inst => {
inst.PropertyA = 2.0;
inst.PropertyB = 2.0;
inst.PropertyC = 2.0;
});
var a = syn.Execute<double>(inst => {
return inst.PropertyA + inst.PropertyB;
});
I know this might sound like an smart a** answer but ... the BEST way to develop threadsafe classes is to actually know about multithreading, about its implications, its intricacies and what does it implies. There's no silver bullet.
First you need a good reason to use it. Threads are a tool, you don't want to hit everything with your new found hammer.
Secondly, learn about the problems of multithreading... deadlocks, race conditions, starvation and so on
Third, make sure is worth it. I'm talking about benefit/cost.
Finally... be prepared to heavy debugging. Debugging multithreaded code is much more difficult than standard old sequential code. Learn some techniques about how to do that.
Seriously... don't try to multithread (in production scenarios I mean) until you know what you're getting yourself into... It can be a huge mistake.
Edit: You should of course know the synchronization primitives of both the operating system and your language of choice (C# under Windows in this case, I guess).
I'm sorry I'm not giving just the code to just make a class threadsafe. That's because it does not exist. A completely threadsafe class will probably just be slower than just avoiding threads and will probably act as a bottleneck to whatever you're doing... effectively undoing whatever you thing you're achieving by using threads.
Bear in mind that the term "thread safe" is not specific; what you're doing here would be more accurately referred to as "synchronization" through the use of a Monitor lock.
That said, the verbosity around synchronized code is pretty much unavoidable. You could cut down on some of the whitespace in your example by turning things like this:
lock (theLock)
{
propertyB = value;
}
into this:
lock (theLock) propertyB = value;
As to whether or not this is the right approach for you we really need more information. Synchronization is just one approach to "thread safety"; immutable objects, semaphores, etc. are all different mechanisms that fit different use-cases. For the simple example you provide (where it looks like you're trying to ensure the atomicity of a get or set operation), then it looks like you've done the right things, but if your code is intended to be more of an illustration than an example then things may not be as simple.
Since no else seems to be doing it, here is some analysis on your specific design.
Want to read any single property? Threadsafe
Want to update to any single property? Threadsafe
Want to read a single property and then update it based on its original value? Not Threadsafe
Thread 2 could update the value between thread 1's read and update.
Want to update two related properties at the same time? Not Threadsafe
You could end up with Property A having thread 1's value and Property B having thread 2's value.
Thread 1 Update A
Thread 2 Update A
Thread 1 Update B
Thread 2 Update B
Want to read two related properties at the same time? Not Threadsafe
Again, you could be interrupted between the first and second read.
I could continue, but you get the idea. Threadsafety is purely based on how you plan to access the objects and what promises you need to make.
You may find the Interlocked class helpful. It contains several atomic operations.
One thing you could do that could help you avoid the extra code is use something like PostSharp to automatically inject those lock statements into your code, even if you had hundreds of them. All you'd need is one attribute attached to the class, and the attribute's implementation which would add the extra locking variables.
As per my comment above - it gets a little hairier if you want simultaneous readers allowed but only one writer allowed. Note, if you have .NET 3.5, use ReaderWriterLockSlim rather than ReaderWriterLock for this type of pattern.
public class ThreadSafeClass
{
private readonly ReaderWriterLock theLock = new ReaderWriterLock();
private double propertyA;
public double PropertyA
{
get
{
theLock.AcquireReaderLock(Timeout.Infinite);
try
{
return propertyA;
}
finally
{
theLock.ReleaseReaderLock();
}
}
set
{
theLock.AcquireWriterLock(Timeout.Infinite);
try
{
propertyA = value;
}
finally
{
theLock.ReleaseWriterLock();
}
}
}
private double propertyB;
public double PropertyB
{
get
{
theLock.AcquireReaderLock(Timeout.Infinite);
try
{
return propertyB;
}
finally
{
theLock.ReleaseReaderLock();
}
}
set
{
theLock.AcquireWriterLock(Timeout.Infinite);
try
{
propertyB = value;
}
finally
{
theLock.ReleaseWriterLock();
}
}
}
public void SomeMethod()
{
theLock.AcquireWriterLock(Timeout.Infinite);
try
{
theLock.AcquireReaderLock(Timeout.Infinite);
try
{
PropertyA = 2.0 * PropertyB;
}
finally
{
theLock.ReleaseReaderLock();
}
}
finally
{
theLock.ReleaseWriterLock();
}
}
}