I haven't had any issues using the same lock for multiple methods so far, but I'm wondering if the following code might actually have issues (performance?) that I'm not aware of:
private static readonly object lockObj = new object();
public int GetValue1(int index)
{
lock(lockObj)
{
// Collection 1 read and/or write
}
}
public int GetValue2(int index)
{
lock(lockObj)
{
// Collection 2 read and/or write
}
}
public int GetValue3(int index)
{
lock(lockObj)
{
// Collection 3 read and/or write
}
}
The 3 methods and the collections are not related in anyway.
In addition, will it be a problem if this lockObj is also used by a singleton (in Instance property) ?
Edit: To clarify my question on using the same lock object in a Singleton class:
private static readonly object SyncObject = new object();
public static MySingleton Instance
{
get
{
lock (SyncObject)
{
if (_instance == null)
{
_instance = new MySingleton();
}
}
return _instance;
}
}
public int MyMethod()
{
lock (SyncObject)
{
// Read or write
}
}
Will this cause issues?
If the methods are unrelated as you state, then use a different lock for each one; otherwise it's inefficient (since there's no reason for different methods to lock on the same object, as they could safely execute concurrently).
Also, it seems that these are instance methods locking on a static object -- was that intended? I have a feeling that's a bug; instance methods should (usually) only lock on instance fields.
Regarding the Singleton design pattern:
While locking can be safe for those, better practice is doing a delayed initialization of a field like this:
private static object sharedInstance;
public static object SharedInstance
{
get
{
if (sharedInstance == null)
Interlocked.CompareExchange(ref sharedInstance, new object(), null);
return sharedInstance;
}
}
This way it's a little bit faster (both because interlocked methods are faster, and because the initialization is delayed), but still thread-safe.
By using the same object to lock on in all of those methods, you are serializing all access to code in all of the threads.
That is... code running GetValue1() will block other code in a different thread from running GetValue2() until it's done. If you add even more code that locks on the same object instance, you'll end up with effectively a single-threaded application at some point.
Shared lock locks other non-related calls
If you use the same lock then locking in one method unnecessarily locks others as well. If they're not related at all than this is a problem since they have to wait for each other. Which they shouldn't.
Bottleneck
This may pose a bottleneck when these methods are frequently called. With separate locks they would run independently, but sharing the same lock it means they must wait for the lock to be released more often as required (actually three times more often).
To create a thread-safe singleton, use this technique.
You don't need a lock.
In general, each lock should be used as little as possible.
The more methods lock on the same thing, the mroe likely you are to end up waiting for it when you don't really need to.
Good question. There are pros and cons of making locks more fine grained vs more coarse grained, with one extreme being a separate lock for each piece of data and the other extreme being one lock for the entire program. As other posts point out, the disadvantage of reusing the same locks is in general you may get less concurrency (though it depends on the case, you may not get less concurrency).
However, the disadvantage of using more locks is in general you make deadlock more likely. There are more ways to get deadlocks the more locks you have involved. For example, acquiring two locks at the same time in separate threads but in the opposite order is a potential deadlock which wouldn't happen if only one lock were involved. Of course sometimes you may fix a deadlock by breaking one lock into two, but usually fewer locks means fewer deadlocks. There's also added code complexity of having more locks.
In general these two factors need to be balanced. It's common to use one lock per class for convenience if it doesn't cause any concurrency issues. In fact, doing so is a design pattern called a monitor.
I would say the best practice is to favor fewer locks for code simplicity's sake and make additional locks if there's a good reason (such as concurrency, or a case where it's more simple or fixes a deadlock).
Related
I have a few general questions when dealing with threads. I have been looking around but haven't really seen any answers to my questions
When dealing with multiple variables in a class you want to be thread safe, are you supposed to have one "lock object" for every variable you want to lock in the class? Like this?
static readonly object lockForVarA = new object();
private float varA;
static readonly object lockForVarB = new object();
private float varB;
Also is this a valid way to handle thread safing a custom type?
public class SomeClass
{
public SomeClass()
{
//Do some kind of work IE load an assembly
}
}
public class SomeOtherClass : BaseClassFiringFromRandomThread
{
static readonly object someClassLock = new object();
SomeClass someClass;
public override void Init()//this is fired from any available thread, can be fired multiple times and even at the same time
{
lock(someClassLock)
{
if(someClass == null)
someClass = new SomeClass();
}
}
}
This code is in the constructor of a class that can be called from any thread at any time
When dealing with multiple variables in a class you want to be thread safe, are you supposed to have one "lock object" for every variable you want to lock in the class?
There are two rules:
Be "fine grained". Have as many locks as possible, one for each variable. Access the variable under its lock every time you use it. Lock as little code as possible to ensure scalability. If you forget to lock a variable, you'll cause a race condition, and if you get the lock ordering wrong, you'll cause a deadlock, so make sure you get it perfect.
Be "coarse-grained". Have just one lock, and put all the critical sections under that lock. Having many locks decreases contention but increases the chance of deadlocks and other errors, so have as few locks as possible, with as much code as possible in each. Of course, this also increases the risk of deadlocks since now there is lots more code inside the locks that can have inversions, and it decreases scalability.
As you have no doubt noticed, the standard advice is completely contradictory. That's because locks are terrible.
My advice: if you don't share variables across threads then you don't need to have any locks at all.
Also is this a valid way to handle thread safing a custom type?
The code looks reasonable so far, but if your intention is to lazy-load some logic then do not write your own threading logic. Just use Lazy<T> and make it do the work. It was written by experts.
Always use the highest-level tool designed by experts that is available to you. Rolling your own threading primitives is a recipe for disaster.
Whatever you do do not take the advice in the other answer that says you must use double checked locking. There are no circumstances in which you must use double-checked locking. Single checked locking is safer, easier, and more likely to be correct. Only use double-checked locking when (1) you have overwhelming empirical evidence that contention is the cause of a measurable, user-impacting performance problem that will be fixed by going low-lock, and (2) you can explain what rules in the C# memory model make double checked locking safe.
If you can't do (1) then you have no reason to do double checked locking, and if you can't do (2), you can't do it with any confidence of safety.
You need to use a double checked lock pattern. There isn't need to acquire your someClassLock lock once someClass has been initialised, and locking it there will just cause unnecessary contention.
if (someClass == null)
{
lock(someClassLock)
{
if (someClass == null)
someClass = new SomeClass();
}
}
You need the inner if block because it is possible a concurrent thread may have created someClass after the first null check but before your lock was acquired.
Of course, you need to also ensure that SomeClass is written in a way that is itself threadsafe, but this will safely ensure that only one instance of someClass is created.
An alternative method is to use Lazy<T> with a suitable LazyThreadSafetyMode.
I have a reasonably simple case of two threads interacting with the same data structure. The threads are hosted in their own responsible classes. Let's say these are class Alfons and class Belzebub:
class Alfons {
public Mutex listMutex = new Mutex();
private void ProcessListInfo()
{
listMutex.WaitOne();
//
// ... Process multi-access list stuff ...
//
listMutex.ReleaseMutex();
}
}
class Belzebub {
private Alfons mCachedAlfonsReference;
private void ProcessListInfoDifferently()
{
mCachedAlfonsReference.listMutex.WaitOne();
//
// ... Process multi-access list stuff in a different fashion ...
//
mCachedAlfonsReference.listMutex.ReleaseMutex();
}
}
My question is whether referencing a Mutex like this can create a concurrency issue OR whether it is recommended practice to do so. Is there a better way of doing this and should I, for example, cache the mutex reference rather than accessing it through a reference.
There would be no concurrency issue - the mutex is supposed to be shared. As per the Mutex MSDN docs
This type is thread safe.
However, I'd say that the data structure itself should synchronize access coming from different threads. If the data structure doesn't support this (e.g., using SyncRoot), encapsulate it and add that feature.
Out of curiosity: which data structure are you using? You might consider using one of the System.Collections.Concurrent collections for lock-free/fine-grained locking solutions. Also, wouldn't using the lock keyword be simpler and less error-prone for your scenario?
Generally, since locking can be tricky and deadlocks will stop all fun, I try to reduce the code that is concerned with the mutex rather than passing it around. Otherwise it can be a headache to figure out which paths lead to a lock.
It may be better to encapsulate the resource and thread critical operations in a class and then:
Lock( this )
{
}
Or see if there is a thread-safe version as suggested by dcastro.
Besides this, be very careful that there is no return (throw, etc) between WaitOne() and ReleaseMutex() otherwise other threads will be locked out indefinitely - lock or a finally with the ReleaseMutex is safer in this respect. As castro pointed out in the comments, it could be another library that raises the exception.
Finally, I am assuming that it is the same resource that is being protected in ProcessListInfo() and ProcessListInfoDifferently(). If these are two different resources that are being protected, then you have extended the likelihood of unnecessary thread contention.
I don't see how caching the mutex reference would make any difference, either way you are still accessing the same object through references, and if you don't do that then it defeats the point of a mutex.
Ok, I've used locks quite a bit, but I've never had this scenario before. I have two different classes that contain code used to modify the same MSAccess database:
public class DatabaseNinja
{
public void UseSQLKatana
{
//Code to execute queries against db.TableAwesome
}
}
public class DatabasePirate
{
public void UseSQLCutlass
{
//Code to execute queries against db.TableAwesome
}
}
This is a problem, because transactions to the database cannot be executed in parallel, and these methods (UseSQLKatana and UseSQLCutlass) are called by different threads.
In my research, I see that it is bad practice to use a public object as a lock object so how do I lock these methods so that they don't run in tandem? Is the answer simply to have these methods in the same class? (That is actually not so simple in my real code)
Well, first off, you could create a third class:
internal class ImplementationDetail
{
private static readonly object lockme = new object();
public static void DoDatabaseQuery(whatever)
{
lock(lockme)
ReallyDoQuery(whatever);
}
}
and now UseSQLKatana and UseSQLCutlass call ImplementationDetail.DoDatabaseQuery.
Second, you could decide to not worry about it, and lock an object that is visible to both types. The primary reason to avoid that is because it becomes difficult to reason about who is locking the object, and difficult to protect against hostile partially trusted code locking the object maliciously. If you don't care about either downside then you don't have to blindly follow the guideline.
The reason it's bad practice to lock on a public object is that you can never be sure who ELSE is locking on that object. Although unlikely, someone else someday can decide that they want to grab your lock object, and do some process that ends up calling your code, where you lock onto that same lock object, and now you have an impossible deadlock to figure out. (It's the same issue for using 'this').
A better way to do this would be to use a public Mutex object. These are much more heavyweight, but it's much easier to debug the issue.
Use a Mutex.
You can create mutex in main class and call Wait method at the beginning of each class (method); then set mutex so when the other method is called it gonna wait for first class to finish.
Ah, remember to release mutex exiting from those methods...
I see two differing questions here:
Why is it a bad idea to lock on a public object?
The idea is that locking on an object restricts access while the lock is maintained - this means none of its members can be accessed, and other sources may not be aware of the lock and attempt to utilise the instance, even trying to acquire a lock themselves, hence causing problems.
For this reason, use a dedicated object instance to lock onto.
How do I lock these methods so that they don't run in tandem?
You could consider the Mutex class; creating a 'global' mutex will allow your classes to operate on the basis of knowing the state of the lock throughout the application. Or, you could use a shared ReaderWriterLockSlim instance, but I wouldn't really recommend the cross-class sharing of it.
You can use a public LOCK object as a lock object. You'll just have to specify that the object you're creating is a Lock object solely used for locking the Ninja and Pirate class.
I have two internal properties that use lazy-loading of backing fields, and are used in a multi-threaded application, so I have implemented a double-checking lock scheme as per this MSDN article
Now, firstly assuming that this is an appropriate pattern, all the examples show creating a single lock object for an instance. If my two properties are independent of each other, would it not be more efficient to create a lock instance for each property?
It occurs to me that maybe there is only one in order to avoid deadlocks or race-conditions. A obvious situation doesn't come to mind, but I'm sure someone can show me one... (I'm not very experienced with multi-threaded code, obviously)
private List<SomeObject1> _someProperty1;
private List<SomeObject2> _someProperty2;
private readonly _syncLockSomeProperty1 = new Object();
private readonly _syncLockSomeProperty2 = new Object();
internal List<SomeObject1> SomeProperty1
{
get
{
if (_someProperty1== null)
{
lock (_syncLockSomeProperty1)
{
if (_someProperty1 == null)
{
_someProperty1 = new List<SomeObject1>();
}
}
}
return _someProperty1;
}
set
{
_someProperty1 = value;
}
}
internal List<SomeObject2> SomeProperty2
{
get
{
if (_someProperty2 == null)
{
lock (_syncLockSomeProperty2)
{
if (_someProperty2 == null)
{
_someProperty2 = new List<SomeObject2>();
}
}
}
return _someProperty2;
}
set
{
_someProperty2 = value;
}
}
If your properties are truly independent, then there's no harm in using independent locks for each of them.
In case the two properties (or their initializers more specifically) are independent of each other, as in the sample code you provided, it makes sense to have two different lock objects. However, when the initialization occurs rarely, the effect will be negligible.
Note that you should protect the setter's code as well. The lock statement imposes a so called memory barrier, which is indispensable especially on multi-CPU and/or multi-core systems to prevent race conditions.
Yes, if they are independent of each other, this would indeed be more efficient, as access to one wont' block access to the other. You're also on the money about the risk of a deadlock if that independence turned out to be false.
The question is, presuming that _someProperty1 = new List<SomeObject1>(); isn't the real code for assigning to _someProperty1 (hardly worth the lazy-load, is it?), then the question is: Can the code that fills SomeProperty1 ever call that which fills SomeProperty2, or vice-versa, through any code-path, no matter how bizarre?
Even if one can call the other, there can't be a deadlock, but if they both can call each other (or 1 call 2, 2 call 3 and 3 call 1, and so on), then a deadlock can definitely happen.
As a rule, I'd start with broad locks (one lock for all locked tasks) and then make the locks narrower as an optimisation as needed. In cases where you have, say, 20 methods which need locking, then judging the safety can be harder (also, you begin to fill memory just with lock objects).
Note that there are two issues with your code also:
First, you don't lock in your setter. Possibly this is fine (you just want your lock to prevent multiple heavy calls to the loading method, and don't actually care if there are over-writes between the set, and the get), possibly this is a disaster.
Second, depending on the CPU running it, double-check as you write it can have issues with read/write reordering, so you should either have a volatile field, or call a memory barrier. See http://blogs.msdn.com/b/brada/archive/2004/05/12/130935.aspx
Edit:
It's also worth considering whether it's really needed at all.
Consider that the operation itself should be thread-safe:
Do a bunch of stuff is done.
Have an object created based on that bunch of stuff.
Assign that object to the local variable.
1 and 2 will only happen on one thread, and 3 is atomic. Therefore, the advantage of locking is:
If performing step 1 and/or 2 above have their own threading issues, and aren't protected from them by their own locks, then locking is 100% necessary.
If it would be disastrous for something to have acted upon a value obtained in step 1 and 2, and then later to do so with step 1 and 2 being repeated, locking is 100% necessary.
Locking will prevent the waste of 1 and 2 being done multiple times.
So, if we can rule out case 1 and 2 as an issue (takes a bit of analysis, but it's often possible), then we've only preventing the waste in case 3 to worry about. Now, maybe this is a big worry. However, if it would rarely come up, and also not be that much of a waste when it did, then the gains of not locking would outweigh the gains of locking.
If in doubt, locking is probably the safer approach, but its possible that just living with the occasional wasted operation is better.
I have a static collections which implements IList interface. This collection is used throughout the application, including adding/removing items.
Due to multithread issue, I wonder what I can do to ensure that the list is modifying one at a time, such as when 1 thread try to add an item, another thread should not do delete item at that time.
I wonder what is difference between lock(this) and lock(privateObject) ? Which one is better in my case?
Thank you.
The lock(this) will lock on the entire instance while lock(privateObject) will only lock that specific instance variable. The second one is the better choice since locking on the entire instance will prevent other threads from being able to do anything with the object.
From MSDN:
In general, avoid locking on a public
type, or instances beyond your code's
control. The common constructs lock
(this), lock (typeof (MyType)), and
lock ("myLock") violate this
guideline:
lock (this) is a problem if the
instance can be accessed publicly.
lock (typeof (MyType)) is a problem if
MyType is publicly accessible.
lock(“myLock”) is a problem since any
other code in the process using the
same string, will share the same lock.
Best practice is to define a private
object to lock on, or a private static
object variable to protect data common
to all instances.
In this particular case, the collection is static which effectively means there is a single instance but that still doesn't change how the lock(this) and lock(privateObject) would behave.
By using lock(this) even in a static collection you are still locking the entire instance. In this case, as soon as thread A acquires a lock for method Foo() all other threads will have to wait to perform any operation on the collection.
Using lock(privateObject) means that as soon as thread A acquires a lock for method Foo() all other threads can perform any other operation except Foo() without waiting. Only when another thread tries to perform method Foo() will it have to wait until thread A has completed its Foo() operation and released the lock.
The lock keyword is a little confusing. The object expression in the lock statement is really just an identification mechanism for creating critical sections. It is not the subject of the lock nor is it in any way guarenteed to be safe for multithreaded operations just because it is referenced by the statement.
So lock(this) is creating a critical section identified by the class containing the currently executing method whereas lock(privateObject) is identified by an object that is (presumably anyway) private to the class. The former is more risky because a caller of your class could inadvertantly define their own critical sections using a lock statement that uses that class instance as the lock object. That could lead to unintended threading problems including, but not limited to, deadlocks and bottlenecks.
You mentioned that you were concerned with multiple threads modifying the collection at the same time. I should point out that you should be equally concerned with threads reading the collection as well even if they are not modifying it. It is likely that you will need some of the same safe guards in place to protect the collection during reads as you would during writes.
Add a private member to the class that methods lock on.
eg.
public class MyClass : IList
{
private object syncRoot = new object();
public void Add(object value)
{
lock(this.syncRoot)
{
// Add code here
}
}
public void Remove(object value)
{
lock(this.syncRoot)
{
// Remove code here
}
}
}
This will ensure that access to the list is syncronized between threads for the adding and removing cases, while maintaining access to the list. This will still let enumerators access the list while another thread can modify it, but that then opens another issue where an enumerator will throw an exception if the collection is modified during the enumeration.