condition_variable

Alessandro Bellina

9 Dec 2009 9 Dec '09

1:50 p.m.

Hello, I am using boost condition_variables to block while waiting for an asynchronous response. I am seeing an error that happens at random whenever I am releasing the lock just before calling notify_one. The error goes away if I keep the lock and release after notify_one. Here is the code: class BlockingRequest{ public: // predicate class class ResponseStatus { public: ResponseStatus( bool& received ) : m_received( received ) { } bool operator()() const { return m_received; } private: bool& m_received; }; //blocking request std::string Request( const std::string& request ) { boost::mutex::scoped_lock lock( mutex ); //perform asynchronous request through network or something else, pass RequestHandler as a handler boost::posix_time::milliseconds timeoutDuration( 10000 ); //wait for 10 seconds if( !condition.timed_wait( lock, timeoutDuration, ResponseStatus( received ) ) ) { throw std::exception( "Request timed out" ); } return receivedMessage; } //some other thread calls this function void RequestHandler( const std::string& message, void* caller ) { BlockingRequest* myRequest = reinterpret_cast< BlockingRequest* >( caller ); boost::mutex::scoped_lock lock( myRequest->mutex ); myRequest->receivedMessage = message; myRequest->received = true; /* if this next line in bold is commented out, code works fine... contrary to Boost documentation. http://www.boost.org/doc/libs/1_41_0/doc/html/thread/synchronization.html#th... */ *lock.unlock();* myRequest->condition.notify_one(); } };

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Steven Watanabe

9 Dec 9 Dec

3:28 p.m.

AMDG Alessandro Bellina wrote:

...

I am using boost condition_variables to block while waiting for an asynchronous response. I am seeing an error that happens at random whenever I am releasing the lock just before calling notify_one. The error goes away if I keep the lock and release after notify_one.

What error do you get? In Christ, Steven Watanabe

Alessandro Bellina

10 Dec 10 Dec

5:38 p.m.

I get an assertion failed error... but I don't know if that would help: "_Pvector == NULL || (((_Myvec *)_Pvector)->_Myfirst <= _Ptr && _Ptr <= ((_Myvec *)_Pvector->_Mylast)" and soon after everything blows up. Here's a thought.. suppose that the thread receiving the response (so the one that calls notify_one), locks the mutex multiple times (without notifying every time)... like this: boost::mutex::scoped_lock lock( mutex ); if( response == "something I care about" ) { lock.unlock(); condition.notify_one(); } and multiple responses did come through. What would the effects be in the condition variable? Why is it that since we already hold a mutex in the requesting thread, the thread handling the response can successfully lock that mutex too? Does the condition.wait call set something in the lock? Thanks, Alessandro On Wed, Dec 9, 2009 at 9:28 AM, Steven Watanabe wrote:

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AMDG

Alessandro Bellina wrote:

...
I am using boost condition_variables to block while waiting for an asynchronous response. I am seeing an error that happens at random whenever I am releasing the lock just before calling notify_one. The error goes away if I keep the lock and release after notify_one.

What error do you get?

In Christ, Steven Watanabe

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Nikolai N Fetissov

6:25 p.m.

...

Why is it that since we already hold a mutex in the requesting thread, the thread handling the response can successfully lock that mutex too? Does the condition.wait call set something in the lock?

Thanks,

Alessandro The idea of wait on a condition variable is to atomically release the mutex and enter some sort of sleep mode, which is later interrupted by a wake-up from a different thread. At that point the mutex is re-aquired. So, yes, condition.wait() unlocks the mutex and waits for notification, then locks the mutex again. -- Nikolai

Alessandro Bellina

9:17 p.m.

Thanks Nikolai, That explains why the second thread can lock that mutex. From what you are saying then, the sleeping thread will wake up and reacquire the lock after notify is called. So the thread that calls notify should not hold that lock when this happens or you could have deadlock right?. Alessandro On Thu, Dec 10, 2009 at 12:25 PM, Nikolai N Fetissov < nikolai-boost@fetissov.org> wrote:

...

...
Why is it that since we already hold a mutex in the requesting thread,

the

...
thread handling the response can successfully lock that mutex too? Does the condition.wait call set something in the lock?

Thanks,

Alessandro

The idea of wait on a condition variable is to atomically release the mutex and enter some sort of sleep mode, which is later interrupted by a wake-up from a different thread. At that point the mutex is re-aquired. So, yes, condition.wait() unlocks the mutex and waits for notification, then locks the mutex again.

-- Nikolai

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Nikolai N Fetissov

9:32 p.m.

...

Thanks Nikolai, That explains why the second thread can lock that mutex. From what you are saying then, the sleeping thread will wake up and reacquire the lock after notify is called. So the thread that calls notify should not hold that lock when this happens or you could have deadlock right?.

Alessandro, The thread calling the 'notify' can, but does not have to, hold a mutex at the time. In theory all the notify() function does is mark the thread sleeping on the condition as runnable. There's no deadlock here. Also google for 'spurious wakeup' to get a firm hold on mutex/cv concepts. Cheers, -- Nikolai

Adam Szeliga

10:11 p.m.

This is scheme which I often use: // Common for both threads boost::mutex mutex; boost::condition condition; // Thread1: boost::xtime waitingTime(set value of waiting time) { boost::mutex::scoped_lock lock(mutex) do { if (some_event occurred) { do something after received event return; // exit point after receive event } } while (condition.timed_wait(lock, waitingTime)); do something after timeout occurred // exit point after timeout } //Thread2 { boost::mutex::scoped_lock lock(mutex); set some event which is been waiting thread1 condition.notify_one(); } Maybe will be useful for you. Adam Nikolai N Fetissov wrote:

...

...
Thanks Nikolai, That explains why the second thread can lock that mutex. From what you are saying then, the sleeping thread will wake up and reacquire the lock after notify is called. So the thread that calls notify should not hold that lock when this happens or you could have deadlock right?.

Alessandro,

The thread calling the 'notify' can, but does not have to, hold a mutex at the time. In theory all the notify() function does is mark the thread sleeping on the condition as runnable. There's no deadlock here. Also google for 'spurious wakeup' to get a firm hold on mutex/cv concepts.

Cheers, -- Nikolai

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Alessandro Bellina

11 Dec 11 Dec

12:03 p.m.

Spurious wake ups? Thanks for posting your example. I don't really know why we need to set "some_event_occurred". Is this the recommended usage? Other than your first iteration (where you need the flag, but that is because you are using a do/while as opposed to a while loop), are we supposed to get spurious wake ups (i.e. will timed_wait return true even if we didn't call notify_one)? Alessandro On Thu, Dec 10, 2009 at 4:11 PM, Adam Szeliga wrote:

...

This is scheme which I often use:

// Common for both threads boost::mutex mutex; boost::condition condition;

// Thread1:

boost::xtime waitingTime(set value of waiting time) {

boost::mutex::scoped_lock lock(mutex) do { if (some_event occurred) { do something after received event return; // exit point after receive event } } while (condition.timed_wait(lock, waitingTime)); do something after timeout occurred // exit point after timeout }

//Thread2

{ boost::mutex::scoped_lock lock(mutex); set some event which is been waiting thread1 condition.notify_one(); }

Maybe will be useful for you. Adam

Nikolai N Fetissov wrote:

...
...
Thanks Nikolai, That explains why the second thread can lock that mutex. From what you are saying then, the sleeping thread will wake up and reacquire the lock after notify is called. So the thread that calls notify should not hold that lock when this happens or you could have deadlock right?.

Alessandro,

The thread calling the 'notify' can, but does not have to, hold a mutex at the time. In theory all the notify() function does is mark the thread sleeping on the condition as runnable. There's no deadlock here. Also google for 'spurious wakeup' to get a firm hold on mutex/cv concepts.

Cheers, -- Nikolai

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Adam Szeliga

8:48 p.m.

- Yes you are right, loop is needed due to spurious wake ups. - timed_wait return true if condition is notified or due to spurious wake ups. We need additional variable for distinguish that. - The flag some_event_occurred is needed in case where conditional.notify_one is called just before calling conditional.timed_wait. Example without flag some_event_occurred: thread 2 -> lock mutex thread 1 -> hold on mutex thread 2 -> condition.notify_one() thread 2 -> unlock mutex thread 1 -> lock mutex thread 1-> hold on condition.timed_wait and waiting up to timeout occurred In this example notify_one "is missing" by thread1 If is it still unclear, please ask more. Regards Adam Alessandro Bellina wrote:

...

Spurious wake ups?

Thanks for posting your example. I don't really know why we need to set "some_event_occurred". Is this the recommended usage? Other than your first iteration (where you need the flag, but that is because you are using a do/while as opposed to a while loop), are we supposed to get spurious wake ups (i.e. will timed_wait return true even if we didn't call notify_one)?

Alessandro

On Thu, Dec 10, 2009 at 4:11 PM, Adam Szeliga mailto:aszeliga.xsoftware@gmail.com> wrote:

This is scheme which I often use:

// Common for both threads boost::mutex mutex; boost::condition condition;

// Thread1:

boost::xtime waitingTime(set value of waiting time) {

boost::mutex::scoped_lock lock(mutex) do { if (some_event occurred) { do something after received event return; // exit point after receive event } } while (condition.timed_wait(lock, waitingTime)); do something after timeout occurred // exit point after timeout }

//Thread2

{ boost::mutex::scoped_lock lock(mutex); set some event which is been waiting thread1 condition.notify_one(); }

Maybe will be useful for you. Adam

Nikolai N Fetissov wrote: >> Thanks Nikolai, >> That explains why the second thread can lock that mutex. From what you are >> saying then, the sleeping thread will wake up and reacquire the lock after >> notify is called. So the thread that calls notify should not hold that >> lock >> when this happens or you could have deadlock right?. >> >> > > Alessandro, > > The thread calling the 'notify' can, but does not have to, hold > a mutex at the time. In theory all the notify() function does is > mark the thread sleeping on the condition as runnable. There's > no deadlock here. Also google for 'spurious wakeup' to get a firm > hold on mutex/cv concepts. > > Cheers, > -- > Nikolai > > _______________________________________________ > Boost-users mailing list > Boost-users@lists.boost.org mailto:Boost-users@lists.boost.org > http://lists.boost.org/mailman/listinfo.cgi/boost-users >

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Adam Szeliga
Alessandro Bellina
Nikolai N Fetissov
Steven Watanabe