I was recently reminded of the UpgradeableReadLock construct C# provides and I'm trying to discern when it really makes sense to use it.
Say, for example, I have a cache of settings that are heavily read by many classes, but periodically need to be updated with a very low frequency based on a set of conditions that aren't necessarily deterministic...
would it make more sense to simply lock like so:
List<Setting> cachedSettings = this.GetCachedSettings( sessionId );
lock(cachedSettings)
{
bool requiresRefresh = cachedSettings.RequiresUpdate();
if(requiresRefresh)
{
// a potentially long operation
UpdateSettings( cachedSettings, sessionId );
}
return cachedSettings;
}
or use an UpgradeableReadLock:
public class SomeRepitory {
private ReaderWriterLockSlim _rw = new ReaderWriterLockSlim();
public List<Setting> GetCachedSettings( string sessionId )
{
_rw.EnterUpgradeableReadLock();
List<Setting> cachedSettings = this.GetCachedSettings( sessionId );
bool requiresRefresh = cachedSettings.RequiresUpdate();
if(requiresRefresh)
{
_rw.EnterWriteLock();
UpdateSettings( cachedSettings, sessionId );
_rw.ExitWriteLock();
}
_rw.ExitUpgradeableReadLock();
return cachedSettings;
}
perhaps what confuses me the most is how we can get away with checking if an update is required outside of the write block. In my example above I am referring to when I check for where a refresh is required, but to simplify I'll use an example from "C# 5.0 In A Nutshell":
while (true)
{
int newNumber = GetRandNum (100);
_rw.EnterUpgradeableReadLock();
if (!_items.Contains (newNumber))
{
_rw.EnterWriteLock();
_items.Add (newNumber);
_rw.ExitWriteLock();
Console.WriteLine ("Thread " + threadID + " added " + newNumber);
}
_rw.ExitUpgradeableReadLock();
Thread.Sleep (100);
}
my understanding is that this allows concurrent reads unless a thread needs to write, but what if two or more threads end up with the same random number and determine !_items.Contains(newNumber)? Given my understanding that this should allow concurrent reads (and correct me if I have misunderstood, of course).. it seems that, as soon as a write lock is obtained, any threads that were concurrently reading would need to be suspended and forced back to the start of _rw.EnterUpgradeableReadLock(); ?
Of course your second approach is better in case of many simultaneous readers and relatively rare write operations. When read lock is acquired (using _rw.EnterUpgradeableReadLock()) by a thread - other threads can also acquire it and read the value simultaneously. When some thread then enters write lock, it waits all reads to complete and then acquires exclusive access to lock object (all other threads trying to execute EnterXXX() operations wait) to update the value. When it releases the lock, other threads can do their job.
First example lock(cachedSettings) blocks all other threads so that only one thread can read the value at a time.
I would recommend in addition use the following pattern:
_rw.EnterUpgradeableReadLock();
try
{
//Do your job
}
finally
{
_rw.ExitUpgradeableReadLock();
}
for all Enter/Exit lock operations. It ensures (with high probability) that if exception happened inside your synchronized code, lock won't remain locked forever.
EDIT:
Answering Martin's comment. If you don't want multiple threads updating the value simultaneously, you need to change your logic to achieve that. For example, using a double-checked lock construct:
if(cachedSettings.RequiresUpdate())
{
_rw.EnterWriteLock();
try
{
if(cachedSettings.RequiresUpdate())
{
UpdateSettings( cachedSettings, sessionId );
}
}
finally
{
_rw.ExitWriteLock();
}
}
This will check if while we were waiting for write lock other thread haven't refreshed that value already. And if value doesn't require refresh anymore - just release the lock.
IMPORTANT: it's very bad to take exclusive lock for long time. So it the UpdateSettings function is long-running, you better execute it outside the lock and implement some additional logic to allow readers read expired value while some thread is refreshing it. I used to implement cache once and it's really complex to make it fast and thread-safe. You better use one of the existing implementations (for example System.Runtime.MemoryCache).
Related
Is it possible to have a conditional thread lock when the underlying condition is not constant?
I have two functions A and B, and a condition to decide which function to execute.
A is thread safe by itself, multiple calls to A can execute simultaneously, B is not, and is Synchronized. But during execution of B the condition can change (from false to true) and therefore all threads executing A at that time will throw errors.
if (condition)
{
A();
}
else
{
B();
}
A - thread safe
B - Synchronized using [MethodImpl(MethodImplOptions.Synchronized)]
Therefore, I am looking for a way to lock A but only when B is running.
Please suggest a way to achieve this.
Some elaborations:
I am creating a cache, and performance is very crucial, thus a blanket lock is not feasible.
Condition is whether or not the requested data is present in the cache.
A() = AddToUpdates() - Executed on a cache hit, just adds to the number of updates for a particular cache key, using a concurrent dictionary.
B() = ProccessUpdates() and EvictLeastPriorityEntry() - Executed on a cache miss, all previous updates will be processed and the underlying data structure storing the ordering of cache entries will be re-arranged.
And then the entry with least priority will be removed.
As mentioned in the accepted answer ReaderWriterLock seems to be the way to go.
Just one problem though,
Let's say, thread1 starts execution and a cache hit occurs, (on the entry with the least priority) meaning the if condition is true and enters the if block. But before calling A(), control is switched to thread2.
thread2 - cache miss occurs, reordering and eviction (Entry which A() from thread1 needed access to) is performed.
Now when controlled is returned to thread1, error will occur.
This is the solution I feel should work:
_lock.EnterReadLock();
if (condition)
{
A();
}
_lock.ExitReadLock();
if (!condition)
{
B();
}
void A()
{
// ....
}
void B()
{
_lock.EnterWriteLock();
// ...
_lock.ExitWriteLock();
}
Will this work?
Thank you.
I possible solution to your problem might be the ReaderWriterLockSlim class. This is a synchronization primitive that allows multiple concurrent readers, or one exclusive writer, but not both of those at the same time.
Use ReaderWriterLockSlim to protect a resource that is read by multiple threads and written to by one thread at a time. ReaderWriterLockSlim allows multiple threads to be in read mode, allows one thread to be in write mode with exclusive ownership of the lock, and allows one thread that has read access to be in upgradeable read mode, from which the thread can upgrade to write mode without having to relinquish its read access to the resource.
Example:
private readonly ReaderWriterLockSlim _lock = new();
void A()
{
_lock.EnterReadLock();
try
{
//...
}
finally { _lock.ExitReadLock(); }
}
void B()
{
_lock.EnterWriteLock();
try
{
//...
}
finally { _lock.ExitWriteLock(); }
}
Your question looks a lot like this:
A() is some read only method, so thread safe. Different execution of A in parallel is OK.
B() is like writing/mutating things that A method uses. So A() becomes not thread safe if executed at same time.
For example B() could write in a List and A() executions read on this list. And you would get exception "InvalidOperationException: Collection Was Modified" thrown from A() .
I advise you to look for "producer/consumer problem" in google and look for the tons of example there are.
But in case you absolutely want to begins B execution while A execution(s) has/have not terminated, you can add checkpoint in A() using Monitor class, it is used to lock a resource and synchronize with other threads. It is more complex though and i would go first for producer/consumer pattern to see if it fill the needs
Some more things:
I would check is the use of BlockingCollection<T> class that may fit your exact need too (and is easy to use)
The use of MethodImplOptions.Synchronized is not recommended because it use public lock. We use usually use private lock (object readonly _lock = new object();) so no one except the maintainer of this object can lock on it, thus preventing dead lock (and preventing other people accusing your code of a bug because other people locked your instance of class without knowing you do the same internally)
I've written a lot of multi-threaded C# code, and I've never had a deadlock in any code I've released.
I use the following rules of thumb:
I tend to use nothing but the lock keyword (I also use other techniques such as reader/writer locks, but sparingly, and only if required for speed).
I use Interlocked.Increment if I am dealing with a long.
I tend to use the smallest granular unit of locking: I only tend to lock around primitive data structures such as long, dictionary or list.
I'm wondering if it's even possible to generate a deadlock if these rules are thumb are consistently followed, and if so, what the code would look like?
Update
I also use these rules of thumb:
Avoid adding a lock around anything that could pause indefinitely, especially I/O operations. If you absolutely have to do so, ensure that absolutely everything within the lock will time out after a set TimeSpan.
The objects I use for locking are always dedicated objects, e.g. object _lockDict = new object(); then lock(_lockDict) { // Access dictionary here }.
Update
Great answer from Jon Skeet. It also confirms why I never get deadlocks as I tend to instinctively avoid nested locks, and even if I do use them, I've always instinctively kept the entry order consistent.
And in response to my comment on tending to use nothing but the lock keyword, i.e. using Dictionary + lock instead of ConcurrentDictionary, Jon Skeet made this comment:
#Contango: That's exactly the approach I'd take too.
I'd go for simple code with locking over "clever" lock-free code every time, until there's evidence that it's causing an issue.
Yes, it's easy to deadlock, without actually accessing any data:
private readonly object lock1 = new object();
private readonly object lock2 = new object();
public void Method1()
{
lock(lock1)
{
Thread.Sleep(1000);
lock(lock2)
{
}
}
}
public void Method2()
{
lock(lock2)
{
Thread.Sleep(1000);
lock(lock1)
{
}
}
}
Call both Method1 and Method2 at roughly the same time, and boom - deadlock. Each thread will be waiting for the "inner" lock, which the other thread has acquired as its "outer" lock.
If you make sure you always acquire locks in the same order (e.g. "never acquire lock2 unless you already own lock1) and release the locks in the reverse order (which is implicit if you're acquiring/releasing with lock) then you won't get that sort of deadlock.
You can still get a deadlock with async code, with just a single thread involved - but that involves Task as well:
public async Task FooAsync()
{
BarAsync().Wait(); // Don't do this!
}
public async Task BarAsync()
{
await Task.Delay(1000);
}
If you run that code from a WinForms thread, you'll deadlock in a single thread - FooAsync will be blocking on the task returned by BarAsync, and the continuation for BarAsync won't be able to run because it's waiting to get back onto the UI thread. Basically, you shouldn't issue blocking calls from the UI thread...
As long as you ever only lock on one thing it's impossible, if one thread tries to lock on multiple locks, then yes. The dining philosophers problem nicely illustrates a simple deadlock caused with simple data.
As the other answers have already shown;
void Thread1Method()
{
lock (lock1)
{
// Do smth
lock (lock2)
{ }
}
}
void Thread2Method()
{
lock (lock2)
{
// Do smth
lock (lock2)
{ }
}
}
Addendum to what Skeet wrote:
The problem normally isn't with "only" two locks... (clearly there could be even with only two locks, but we want to play in Hard mode :-) )...
Let's say that in your program there are 10 lockable resources... Let's call them a1...a10. You must be sure that you'll always lock those in the same order, even for subsets of them... If a method needs a3, a5 and a7, and another methods needs a4, a5, a7, you must be sure that both will try locking them in the "right" order. For simplicity sake in this case the order is clear: a1->a10.
Normally lock objects aren't numbered, and/or they aren't taken in a single method... For example:
void MethodA()
{
lock (Lock1)
{
CommonMethod();
}
}
void MethodB()
{
lock (Lock3)
{
CommonMethod();
}
}
void CommonMethod()
{
lock (Lock2)
{
}
}
void MethodC()
{
lock (Lock1)
{
lock (Lock2)
{
lock (Lock3)
{
}
}
}
}
Here, even with the Lock* numbered, it isn't immediately clear that the locks could be taken in the wrong order (MethodB+CommonMethod take Lock3+Lock2, while MethodC takes Lock1+Lock2+Lock3)... It isn't immediately clear and we are playing with three very big advantages: we are speaking of deadlock, so we are looking for them, the locks are numbered and the whole code is around 30 lines.
I have a function in C# that can be called multiple times from multiple threads and I want it to be done only once so I thought about this:
class MyClass
{
bool done = false;
public void DoSomething()
{
lock(this)
if(!done)
{
done = true;
_DoSomething();
}
}
}
The problem is _DoSomething takes a long time and I don't want many threads to wait on it when they can just see that done is true.
Something like this can be a workaround:
class MyClass
{
bool done = false;
public void DoSomething()
{
bool doIt = false;
lock(this)
if(!done)
doIt = done = true;
if(doIt)
_DoSomething();
}
}
But just doing the locking and unlocking manually will be much better.
How can I manually lock and unlock just like the lock(object) does? I need it to use same interface as lock so that this manual way and lock will block each other (for more complex cases).
The lock keyword is just syntactic sugar for Monitor.Enter and Monitor.Exit:
Monitor.Enter(o);
try
{
//put your code here
}
finally
{
Monitor.Exit(o);
}
is the same as
lock(o)
{
//put your code here
}
Thomas suggests double-checked locking in his answer. This is problematic. First off, you should not use low-lock techniques unless you have demonstrated that you have a real performance problem that is solved by the low-lock technique. Low-lock techniques are insanely difficult to get right.
Second, it is problematic because we don't know what "_DoSomething" does or what consequences of its actions we are going to rely on.
Third, as I pointed out in a comment above, it seems crazy to return that the _DoSomething is "done" when another thread is in fact still in the process of doing it. I don't understand why you have that requirement, and I'm going to assume that it is a mistake. The problems with this pattern still exist even if we set "done" after "_DoSomething" does its thing.
Consider the following:
class MyClass
{
readonly object locker = new object();
bool done = false;
public void DoSomething()
{
if (!done)
{
lock(locker)
{
if(!done)
{
ReallyDoSomething();
done = true;
}
}
}
}
int x;
void ReallyDoSomething()
{
x = 123;
}
void DoIt()
{
DoSomething();
int y = x;
Debug.Assert(y == 123); // Can this fire?
}
Is this threadsafe in all possible implementations of C#? I don't think it is. Remember, non-volatile reads may be moved around in time by the processor cache. The C# language guarantees that volatile reads are consistently ordered with respect to critical execution points like locks, and it guarantees that non-volatile reads are consistent within a single thread of execution, but it does not guarantee that non-volatile reads are consistent in any way across threads of execution.
Let's look at an example.
Suppose there are two threads, Alpha and Bravo. Both call DoIt on a fresh instance of MyClass. What happens?
On thread Bravo, the processor cache happens to do a (non-volatile!) fetch of the memory location for x, which contains zero. "done" happens to be on a different page of memory which is not fetched into the cache quite yet.
On thread Alpha at the "same time" on a different processor DoIt calls DoSomething. Thread Alpha now runs everything in there. When thread Alpha is done its work, done is true and x is 123 on Alpha's processor. Thread Alpha's processor flushes those facts back out to main memory.
Thread bravo now runs DoSomething. It reads the page of main memory containing "done" into the processor cache and sees that it is true.
So now "done" is true, but "x" is still zero in the processor cache for thread Bravo. Thread Bravo is not required to invalidate the portion of the cache that contains "x" being zero because on thread Bravo neither the read of "done" nor the read of "x" were volatile reads.
The proposed version of double-checked locking is not actually double-checked locking at all. When you change the double-checked locking pattern you need to start over again from scratch and re-analyze everything.
The way to make this version of the pattern correct is to make at least the first read of "done" into a volatile read. Then the read of "x" will not be permitted to move "ahead" of the volatile read to "done".
You can check the value of done before and after the lock:
if (!done)
{
lock(this)
{
if(!done)
{
done = true;
_DoSomething();
}
}
}
This way you won't enter the lock if done is true. The second check inside the lock is to cope with race conditions if two threads enter the first if at the same time.
BTW, you shouldn't lock on this, because it can cause deadlocks. Lock on a private field instead (like private readonly object _syncLock = new object())
The lock keyword is just syntactic sugar for the Monitor class. Also you could call Monitor.Enter(), Monitor.Exit().
But the Monitor class itself has also the functions TryEnter() and Wait() which could help in your situation.
I know this answer comes several years late, but none of the current answers seem to address your actual scenario, which only became apparent after your comment:
The other threads don't need to use any information generated by ReallyDoSomething.
If the other threads don't need to wait for the operation to complete, the second code snippet in your question would work fine. You can optimize it further by eliminating your lock entirely and using an atomic operation instead:
private int done = 0;
public void DoSomething()
{
if (Interlocked.Exchange(ref done, 1) == 0) // only evaluates to true ONCE
_DoSomething();
}
Furthermore, if your _DoSomething() is a fire-and-forget operation, then you might not even need the first thread to wait for it, allowing it to run asynchronously in a task on the thread pool:
int done = 0;
public void DoSomething()
{
if (Interlocked.Exchange(ref done, 1) == 0)
Task.Factory.StartNew(_DoSomething);
}
Ok first I must preface this question with a disclaimer, I'm really new to threading so this may be a 'newbie' question but I searched google and couldn't find an answer. As I understand it a critical section is code that can be accessed by two or more threads, the danger being one thread will overwrite a value before the other is finished and vice versa. What can you do about changes made outside of your class for example, I have a line monitoring program:
int currentNumber = provider.GetCurrentNumber();
if(provider.CanPassNumber(false, currentNumber))
{
currentNumber++;
provider.SetNumber(currentNumber);
}
and on another thread I have something like this:
if(condition)
provider.SetNumber(numberToSet);
Now I'm afraid that in the first function I get currentNumber which is 5, right after that on another thread the number is set to 7 and then it rewrites the 7 to 6, ignoring the change made by the thread that set it to 7.
Is there anyway to lock provider.SetNumber until the first function finishes? The critical section is basically the currentNumber which can be changed by many places in the program.
I hope I made myself clear, if not let me know and I will try to explain myself better.
EDIT:
Also I made the functions really short for the example. In reality the function is much longer and makes changes to currentNumber many times so I don't really want to put a lock around the entire function. If I lock every call to provider.SetNumber and release it after I finish it can change during the time it is released before I lock it again to call provider.SetNumber. Honestly I'm also worried about locking the entire function because of performance and deadlock.
Rather than using the lock() keywords I'd suggested seeing if you can use the Interlocked class which is designed for small operations. It's got much less overhead than lock, in fact can be down to a single CPU instruction on some CPUs.
There are a couple of methods of interest for you, Exchange and Read, both of which are thread safe.
You want to look into the Lock keyword. Also you might want to this tutorial to Threading in C#.
As Filip said, lock is useful here.
Not only should you lock on provider.SetNumber(currentNumber), you also need to lock on any conditional that the setter depends on.
lock(someObject)
{
if(provider.CanPassNumber(false, currentNumber))
{
currentNumber++;
provider.SetNumber(currentNumber);
}
}
as well as
if(condition)
{
lock(someObject)
{
provider.SetNumber(numberToSet);
}
}
If condition is reliant on numberToSet, you should take the lock statement around the whole block. Also note that someObject must be the same object.
You can use the lock statement, to enter a critical section with mutual exclusion. The lock will use the object's reference to differentiate one critical section from another, you must have the same reference for all your lock if it accesses to the same elements.
// Define an object which can be locked in your class.
object locker = new object();
// Add around your critical sections the following :
lock (locker) { /* ... */ }
That will change your code to :
int currentNumber = provider.GetCurrentNumber();
lock (locker)
{
if(provider.CanPassNumber(false, currentNumber))
{
currentNumber++;
provider.SetNumber(currentNumber);
}
}
And :
if(condition)
{
lock (locker)
{
provider.SetNumber(numberToSet);
}
}
In your SetNumber method you can simply use a lock statement:
public class MyProvider {
object numberLock = new object();
...
public void SetNumber(int num) {
lock(numberLock) {
// Do Stuff
}
}
}
Also, note that in your example currentNumber is a primitive (int), which means that variable's value won't be overwritten should your provider's actual data member's value change.
Well first of im not so good with threading but a critical section is a part of your code that can only be accessed my one thread at a time not the other way around..
To create a critical section is easy
Lock(this)
{
//Only one thread can run this at a time
}
note: that this should be replaced with some internal object...
I'm using simple locking in C# using the lock statement. Is there any way to determine how many other threads are waiting to get a lock on the object? I basically want to limit the number of threads that are waiting for a lock to 5. My code would throw an exception if a sixth thread needs to get a lock.
This can be easily accomplished via the Semaphore class. It will do the counting for you. Notice in the code below that I use a semaphore to do a non-blocking check of the number of threads waiting for the resource and then I use a plain old lock to actually serialize access to that resource. An exception is thrown if there are more than 5 threads waiting for the resource.
public class YourResourceExecutor
{
private Semaphore m_Semaphore = new Semaphore(5, 5);
public void Execute()
{
bool acquired = false;
try
{
acquired = m_Semaphore.WaitOne(0);
if (!acquired)
{
throw new InvalidOperationException();
}
lock (m_Semaphore)
{
// Use the resource here.
}
}
finally
{
if (acquired) m_Semaphore.Release();
}
}
}
There is one notable variation of this pattern. You could change the name of the method to TryExecute and have it return a bool instead of throwing an exception. It is completely up to you.
Remember that the object used in the lock expression is not the subject of the lock. It merely serves as an identifier for a synchronized block of code. Any code blocks that acquire locks using the same object will effectively be serialized. It is the code block that is being "locked", not the object used in the lock expression
The lock statement is a shortcut for Monitor.Enter and Monitor.Exit. I do not think, that you have a chance to get the number of waiting objects.
You can use a simple shared counter(integer) that increments before the lock statement. If the value is equal to 5 then have your thread avoid the lock statement. The challenge however is that you will need to lock the counter to ensure the increment operation is atomic.
No, lock() uses the Monitor class and that has no member for finding out the nr of queued threads.
You can specify a time-out.
And frankly, throwing an Exception when a queue fills up sounds like a bad idea.