Do I need to declare an static Object and use lock on it like
private static readonly Object padlock = new Object()
public static Test()
{
lock(padlock) {
// Blah Blah Blah
}
}
(Your code wouldn't currently compile, by the way - Readonly should be readonly, and you need to give padlock a type.)
It depends on what you're doing in the method. If the method doesn't use any shared data, or uses it in a way which is already safe, then you're fine.
You generally only need to lock if you're accessing shared data in an otherwise non-thread-safe way. (And all access to that shared data needs to be done in a thread-safe way.)
Having said that, I should point out that "thread safe" is a pretty vague term. Eric Lippert has a great blog post about it... rather than trying to come up with a "one size fits all" approach, you should think about what you're trying to protect against, what scenarios you're anticipating etc.
Jon is right; it is really not clear what you are asking here. The way I would interpret your question is:
If I have some shared state that needs to be made thread-safe by locking it, am I required to declare a private static object as the lock object?
The answer to that question is no, you are not required to do so. However, doing so is a really good idea, so you should do so even if you are not required.
You might think, well, there are lots of objects I could use. If the object I am locking is a reference type, I could use it. Or I could use the Type object associated with the containing class.
The problem with those things as locks is it becomes difficult to track down every possible bit of code that could be using the thing as a lock. Therefore it becomes difficult to analyze the code to ensure that there are no deadlocks due to lock ordering issues. And therefore, you are much more likely to get deadlocks. Having a dedicated lock object makes it much easier; you know that every single usage of that object is for the purposes of locking, and you can then understand what is going on inside those locks.
This is particularly true if you ever have untrusted, hostile code running in your appdomain. Locking a type object requires no particular permissions; what stops hostile code from locking all the types and never unlocking them? Nothing, that's what.
Related
This question already has answers here:
C# lock statement, what object to lock on?
(4 answers)
Closed 8 years ago.
I have seen few different ways of using a particular object for lock construct.
Have a dedicated private static local variable and lock on that
private static object syncObj = new object();
...
lock(syncObj)
{
}
Have a dedicated private local variable and lock on that
private object syncObj = new object();
...
lock(syncObj)
{
}
Another one is to use the object itself.
private List<MyClass> SomeObjects = new List<MyClass>();
....
lock(SomeObjects)
{
}
One approach is to use the type of a particular object.
private List<MyClass> SomeObjects = new List<MyClass>();
...
lock(SomeObjects.GetType())
{
}
Another possibility is using lock(this) but general recommendation seem to try to avoid using this.
My questions.
1. Are there any other ways of using objects for locks?
2. How can I decide of what approach should be used for a particular scenario?
If Marc Gravell's answer to C# lock statement, what object to lock on? were the top vote-getter (and it should have been, IMHO), I would happily have voted to close this question as a duplicate.
But, it's not. And this question does have some minor differences in context. So…
Taking your five examples in particular:
Have a dedicated private static local variable and lock on that
This works well for scenarios where you have some static member that needs to be synchronized. A common implementation convention in .NET is to make all static members thread-safe, and for types which are not themselves specifically targeting multi-threaded code, to not bother making instance members thread-safe.
Note that the declaration should be static readonly, to ensure that the locking object remains the same through the lifetime of the program.
Have a dedicated private local variable and lock on that
This is better for protecting instance members, in a class that is specifically supposed to be thread-safe. While a static lock object would also work, that can be needlessly contentious. That is, private instance members are generally safe from other instances of the object touching them, as classes usually only operate on their own instance members, not those of other instances.
A static lock object would require all threads operating on any instance to synchronize their execution, when it would be safe for threads using different instances to operate concurrently.
As with the static lock object, make the field readonly.
Another one is to use the object itself.
I tend to try to avoid doing this. If the object is maintained privately, and you are sure that the reference is never known to any other code other than your own and that within the object itself, it can be safe enough. But it can be risky, as you can't be sure even if the object is currently never used elsewhere that the code will never change such that it later.
If the reference were to become available outside your own class, then it's possible some other code could also lock on the object in an inopportune way. The worst would be if it acquired some other lock, then tried to lock on that object, while your own code tried to acquire the other lock having already locked on that object. Deadlock. Less bad is simply increasing thread contention on the lock. The code would still work, but may not run as well.
One approach is to use the type of a particular object
This combines the worst of the above: publicly available reference, static member. Strongly advised against.
Another possibility is using lock(this) but general recommendation seem to try to avoid using this.
This is just a variation on #3, except that you are practically guaranteed that the reference will be used by some other code. Not advised.
You can, in the sense that "it is possible", use any reference with a lock statement (i.e. any class value…you definitely should not use any struct value, as suggested in the answer to the question which Preston has suggested as a duplicate for this one). However, there are lots of ways to get this wrong.
IMHO, the best policy is to keep it simple: match the static/instance declaration of whatever it is you're trying to protect (either a whole object, or in some cases specific operations on a specific object), always make the field readonly, and be done with it.
Specific Answers Only Please! I'm decently familiar with the better(best) practices around collection locking, thread safety etc. Just want some answers / ideas around this specific scenario.
We have some legacy code of the type:
public class GodObject
{
private readonly Dictionary<string, string> _signals;
//bunch of methods accessing the dictionary
private void SampleMethod1()
{
lock(_signals)
{
//critical code section 1
}
}
public void SampleMethod2()
{
lock(_signals)
{
//critical code section 2
}
}
}
All access to the dictionary is inside such lock statements. We're getting some bugs which could be explained if the locking was not explicitly working - meaning 2 or more threads getting simultaneous access to the dictionary.
So my question is this - is there any scenario where the critical sections could be simultaneously accessed by multiple threads?? To me, it should not be possible, since the reference is readonly, it's not as though the object could be changing, and most of the issues around the lock() are around deadlocks rather than syncronization not happening. But maybe i'm missing some nuance or something glaring?
This is running in a long running windows service .NET Framework 3.5.
There are three problems I can imagine occurring outside the code you posted:
Somebody might access the dictionary without locking on it. Using lock on an object will prevent anyone else from using lock on the same object at the same time, but it won't do anything to prevent other threads from using the object without locking on it. Note that because it would not have been overly difficult to have written Dictionary [and for that matter List] in such a way as to allow safe simultaneous use by multiple readers and one writer that only adds information, some people may assume that read methods don't need locking. Unfortunately, that assumption is false: Microsoft could have added such thread safety fairly cheaply, but didn't.
As Servy suggested, someone might be assuming that the the collection won't change between calls to two independent methods.
If some code which acquires a lock assumes a collection isn't going to change while the lock is held, but then calls some outside method while holding the lock, it's possible that the outside method could change the object despite the lock being held.
Unless the object which owns the dictionary keeps all references to itself, so that no outside code ever gets a reference to the dictionary, I think the first of these problems is perhaps the most likely. The other two problems can also occur sometimes, however.
I have the following Lock statement:
private readonly object ownerLock_ = new object();
lock (ownerLock_)
{
}
Should I use volatile keyword for my lock variable?
private readonly volatile object ownerLock_ = new object();
On MSDN I saw that it usually used for a field that is accessed without locking, so if I use Lock I don't need to use volatile?
From MSDN:
The volatile modifier is usually used for a field that is accessed by
multiple threads without using the lock statement to serialize access.
If you're only ever accessing the data that the lock "guards" while you own the lock, then yes - making those fields volatile is superfluous. You don't need to make the ownerLock_ variable volatile either. (You haven't currently shown any actual code within the lock statement, which makes it hard to talk about in concrete terms - but I'm assuming you'd actually be reading/modifying some data within the lock statement.)
volatile should be very rarely used in application code. If you want lock-free access to a single variable, Interlocked is almost always simpler to reason about. If you want lock-free access beyond that, I would almost always start locking. (Or try to use immutable data structures to start with.)
I'd only expect to see volatile within code which is trying to build higher level abstractions for threading - so within the TPL codebase, for example. It's really a tool for experts who really understand the .NET memory model thoroughly... of whom there are very few, IMO.
If something is readonly it's thread-safe, period. (Well, almost. An expert might be able to figure out how to get a NullReferenceException on your lock statement, but it wouldn't be easy.) With readonly you don't need volatile, Interlocked, or locking. It's the ideal keyword for multi-threading, and you should use it where ever you can. It works great for a lock object where its big disadvantage (you can't change the value) doesn't matter.
Also, while the reference is immutable, the object referenced may not be. "new object()" is here, but if it was a List or something else mutable--and not thread-safe--you would want to lock the reference (and all other references to it, if any) to keep the object from changing in two threads at once.
Is it necessary to protect access to a single variable of a reference type in a multi-threaded application? I currently lock that variable like this:
private readonly object _lock = new object();
private MyType _value;
public MyType Value
{
get { lock (_lock) return _value; }
set { lock (_lock) _value = value; }
}
But I'm wondering if this is really necessary? Isn't assignment of a value to a field atomic? Can anything go wrong if I don't lock in this case?
P.S.: MyType is an immutable class: all the fields are set in the constructor and don't change. To change something, a new instance is created and assigned to the variable above.
Being atomic is rarely enough.
I generally want to get the latest value for a variable, rather than potentially see a stale one - so some sort of memory barrier is required, both for reading and writing. A lock is a simple way to get this right, at the cost of potentially losing some performance due to contention.
I used to believe that making the variable volatile would be enough in this situation. I'm no longer convinced this is the case. Basically I now try to avoid writing lock-free code when shared data is involved, unless I'm able to use building blocks written by people who really understand these things (e.g. Joe Duffy).
There is the volatile keyword for this. Whether it's safe without it depends on the scenario. But the compiler can do funny stuff, such as reorganize order of operation. So even read/write to one field may be unsafe.
It can be an issue. It's not just the assignment itself you have to be concerned with. Due to caching, concurrent threads might see an old version of the object if you don't lock. So whether a lock is necessary will depend on precisely how you use it, and you don't show that.
Here's a free, sample chapter of "Concurrent Programming in Windows" which explains this issue in detail.
it all depends on whether the property will be accessed by multiple threads. and some variable is said to be atomic operation, in this atomic operation case, no need to use lock. sorry for poor english.
in you case, immutable, i think lock is not necessary.
In a multi-threaded program running on a multi-cpu machine do I need to access shared state ( _data in the example code below) using volatile read/writes to ensure correctness.
In other words, can heap objects be cached on the cpu?
Using the example below and assuming multi-threads will access the GetValue and Add methods, I need ThreadA to be able to add data (using the Add Method) and ThreadB to be able to see/get that added data immediately (using the GetValue method). So do I need to add volatile reads/writes to _data to ensure this? Basically I don’t want to added data to be cached on ThreadA’s cpu.
/ I am not Locking (enforcing exclusive thread access) as the code needs to be ultra-fast and I am not removing any data from _data so I don’t need to lock _data.
Thanks.
**** Update ****************************
Obviously you guys think going lock-free using this example is bad idea. But what side effects or exceptions could I face here?
Could the Dictionary type throw an exception if 1 thread is iterating the values for read and another thread is iterating the values for update? Or would I just experience “dirty reads” (which would be fine in my case)?
**** End Update ****************************
public sealed class Data
{
private volatile readonly Dictionary<string, double> _data = new Dictionary<string, double>();
public double GetVaule(string key)
{
double value;
if (!_data.TryGetValue(key, out value))
{
throw new ArgumentException(string.Format("Key {0} does not exist.", key));
}
return value;
}
public void Add(string key, double value)
{
_data.Add(key, value);
}
public void Clear()
{
_data.Clear();
}
}
Thanks for the replies. Regarding the locks, the methods are pretty much constantly called by mulitple threads so my problem is with contested locks not the actual lock operation.
So my question is about cpu caching, can heap objects (the _data instance field) be cached on a cpu? Do i need the access the _data field using volatile reads/writes?
/Also, I am stuck with .Net 2.0.
Thanks for your help.
The MSDN docs for Dictionary<TKey, TValue> say that it's safe for multiple readers but they don't give the "one writer, multiple readers" guarantee that some other classes do. In short, I wouldn't do this.
You say you're avoiding locking because you need the code to be "ultra-fast" - have you tried locking to see what the overhead is? Uncontested locks are very cheap, and when the lock is contested that's when you're benefiting from the added safety. I'd certainly profile this extensively before deciding to worry about the concurrency issues of a lock-free solution. ReaderWriterLockSlim may be useful if you've actually got multiple readers, but it sounds like you've got a single reader and a single writer, at least at the moment - simple locking will be easier in this case.
I think you may be misunderstanding the use of the volatile keyword (either that or I am, and someone please feel free to correct me). The volatile keyword guarantees that get and set operations on the value of the variable itself from multiple threads will always deal with the same copy. For instance, if I have a bool that indicates a state then setting it in one thread will make the new value immediately available to the other.
However, you never change the value of your variable (in this case, a reference). All that you do is manipulate the area of memory that the reference points to. Declaring it as volatile readonly (which, if my understanding is sound, defeats the purpose of volatile by never allowing it to be set) won't have any effect on the actual data that's being manipulated (the back-end store for the Dictionary<>).
All that being said, you really need to use a lock in this case. Your danger extends beyond the prospect of "dirty reads" (meaning that what you read would have been, at some point, valid) into truly unknown territory. As Jon said, you really need proof that locking produces unacceptable performance before you try to go down the road of lockless coding. Otherwise that's the epitome of premature optimization.
The problem is that your add method:
public void Add(string key, double value)
{
_data.Add(key, value);
}
Could cause _data to decide to completely re-organise the data it's holding - at that point a GetVaule request could fail in any possible way.
You need a lock or a different data structure / data structure implementation.
I don't think volatile can be a replacement of locking if you start calling methods on it. You are guaranteeing that the thread A and thread B sees the same copy of the dictionary, but you can still access the dictionary simultaneously. You can use multi-moded locks to increase concurrency. See ReaderWriterLockSlim for example.
Represents a lock that is used to
manage access to a resource, allowing
multiple threads for reading or
exclusive access for writing.
The volatile keyword is not about locking, it is used to indicate that the value of the specified field might be changed or read by different thread or other thing that can run concurrently with your code. This is crucial for the compiler to know, because many optimization processes involve caching the variable value and rearranging the instructions. The volatile keyword will tell the compiler to be "cautious" when optimizing those instructions that reference to volatile variable.
For multi-thread usage of dictionary, there are many ways to do. The simplest way is using lock keyword, which has adequate performance. If you need higher performance, you might need to implement your own dictionary for your specific task.
Volatile is not locking, it has nothing to do with synchronization. It's generally safe to do lock-free reads on read-only data. Note that just because you don't remove anything from _data, you seem to call _data.Add(). That is NOT read-only. So yes, this code will blow up in your face in a variety of exciting and difficult to predict ways.
Use locks, it's simple, it's safer. If you're a lock-free guru (you're not!), AND profiling shows a bottleneck related to contention for the lock, AND you cannot solve the contention issues via partitioning or switching to spin-locks THEN AND ONLY THEN can you investigate a solution to get lock-free reads, which WILL involve writing your own Dictionary from scratch and MAY be faster than the locking solution.
Are you starting to see how far off base you are in your thinking here? Just use a damn lock!