BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Need help with Mutex          BorlandTalk.com Forum Index -> C++ Builder (Native API) View previous topic :: View next topic   Author Message SubZero Guest Posted: Sat Apr 28, 2007 9:34 pm    Post subject: Need help with Mutex Hello, I used to have an application with the following functions for thread sync'in. void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { criticalSection->Acquire(); }; void inline __fastcall releaseCriticalSection(TCriticalSection *criticalSection) { criticalSection->Release(); }; To detect a deadlock, I implemented the below: void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { HANDLE hMutex; // Create a mutex with no initial owner. hMutex = CreateMutex( NULL, // no security attributes true, // initially owned String((int)(void*)criticalSection).c_str()); // name of mutex DWORD dwWaitResult; // Request ownership of mutex. if(hMutex) { dwWaitResult = WaitForSingleObject( hMutex, // handle to mutex 5000L); // five-second time-out interval switch(dwWaitResult) { // Cannot get mutex ownership due to time-out. case WAIT_TIMEOUT: throw Exception("BUG FOUND, CHECK CALL STACK!"); // Got ownership of the abandoned mutex object. case WAIT_ABANDONED: break; } } criticalSection->Acquire(); }; void inline __fastcall releaseCriticalSection(TCriticalSection *criticalSection) { criticalSection->Release(); HANDLE hMutex; // Create a mutex with no initial owner. hMutex = OpenMutex( MUTEX_ALL_ACCESS, false, String((int)(void*)criticalSection).c_str() ); // name of mutex if(hMutex) { DWORD dwWaitResult; // Request ownership of mutex. dwWaitResult = WaitForSingleObject( hMutex, // handle to mutex 5000L); // five-second time-out interval switch(dwWaitResult) { // The thread got mutex ownership. case WAIT_OBJECT_0: // Release ownership of the mutex object. if(!ReleaseMutex(hMutex)) { // Deal with error. } break; // Cannot get mutex ownership due to time-out. case WAIT_TIMEOUT: break; // Got ownership of the abandoned mutex object. case WAIT_ABANDONED: break; } } }; which does not work for no obvious reason. It simply raises the exception when there is no reason! The Critical section variable is -1, 0 at that time! Please help! Best Regards, SZ Back to top Dennis Jones Guest Posted: Sat Apr 28, 2007 10:12 pm    Post subject: Re: Need help with Mutex "SubZero" <g_ates (AT) hotmail (DOT) com> wrote in message news:4633777f$1 (AT) newsgroups (DOT) borland.com... Quote: void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { HANDLE hMutex; // Create a mutex with no initial owner. hMutex = CreateMutex( NULL, // no security attributes true, // initially owned String((int)(void*)criticalSection).c_str()); // name of mutex DWORD dwWaitResult; // Request ownership of mutex. if(hMutex) { dwWaitResult = WaitForSingleObject( hMutex, // handle to mutex 5000L); // five-second time-out interval Perhaps I am mistaken, but it looks to me like the problem is obvious...your comment says, "Create a mutex with no initial owner", but the code creates it owned (conflicts with the comment). Then you attempt to wait for the mutex, and 5 seconds later, due to timeout, the exception is thrown. - Dennis Back to top Bob Gonder Guest Posted: Sat Apr 28, 2007 10:31 pm    Post subject: Re: Need help with Mutex SubZero wrote: Quote: which does not work for no obvious reason. It simply raises the exception when there is no reason! What exception would that be? Why would you want to use _both_ mutex and CriticalSection ? I understand you want a timeout, so you use mutex, but that should be enough. I think it would be better to have a global handle for the mutex so you aren't constantly creating and opening the mutex (which you don't use CloseHandle on, so are leaking resources). Your lockCriticalSection() calls CreateMutex() with true, which is not correct for this. Think what happens if 2 threads (or applications) try to lock at the same time. "Create with ownership" should fail for one of them. Furthermore: <help> The thread that owns a mutex can specify the same mutex in repeated wait function calls without blocking its execution. Typically, you would not wait repeatedly for the same mutex, but this mechanism prevents a thread from deadlocking itself while waiting for a mutex that it already owns. However, to release its ownership, the thread must call ReleaseMutex once for each time that the mutex satisfied a wait. </help> What this means is your CreateMutex(true) satisfies one "ownership", and the wait() satisfies a second, but you only release once. IMO you should CreateMutex(false) in application startup, with CloseHandle() at shutdown. Then the Lock() and and Release() would just use the global mutex handle. BTW, since your code should not reach releaseLock() without having aquired a lock, releaseLock() should not wait(), but simply ReleaseMutex(). Back to top Bob Gonder Guest Posted: Sat Apr 28, 2007 10:36 pm    Post subject: Re: Need help with Mutex Dennis Jones wrote: Quote: Perhaps I am mistaken, but it looks to me like the problem is obvious...your comment says, "Create a mutex with no initial owner", but the code creates it owned (conflicts with the comment). Then you attempt to wait for the mutex, and 5 seconds later, due to timeout, the exception is thrown. The CreateMutex(true) returns with ownership. The wait() satisfies with pre-ownership to eliminate deadlock. Drawback is now the mutex is owned twice and should be released twice. Back to top SubZero Guest Posted: Sat Apr 28, 2007 10:46 pm    Post subject: Re: Need help with Mutex Ok. The original functions will be used with only TCriticalSection (single line). The reason for all the code is that it is needed for detecting the deadlock. I want to raise an exception so that MadExcept can show the stack! Hope you understand now. Sorry for the confusion. I cannot create the mutexes at startup because there are countless criticalsections. It must work within these two functions with named mutexes. Best Regards, SZ "Bob Gonder" <notbg (AT) notmindspring (DOT) invalid>, iletisinde sunu yazdi, news:aev6339dmkoh5ih93feqeofs77v59f6iic (AT) 4ax (DOT) com... Quote: SubZero wrote: which does not work for no obvious reason. It simply raises the exception when there is no reason! What exception would that be? Why would you want to use _both_ mutex and CriticalSection ? I understand you want a timeout, so you use mutex, but that should be enough. I think it would be better to have a global handle for the mutex so you aren't constantly creating and opening the mutex (which you don't use CloseHandle on, so are leaking resources). Your lockCriticalSection() calls CreateMutex() with true, which is not correct for this. Think what happens if 2 threads (or applications) try to lock at the same time. "Create with ownership" should fail for one of them. Furthermore: help The thread that owns a mutex can specify the same mutex in repeated wait function calls without blocking its execution. Typically, you would not wait repeatedly for the same mutex, but this mechanism prevents a thread from deadlocking itself while waiting for a mutex that it already owns. However, to release its ownership, the thread must call ReleaseMutex once for each time that the mutex satisfied a wait. /help What this means is your CreateMutex(true) satisfies one "ownership", and the wait() satisfies a second, but you only release once. IMO you should CreateMutex(false) in application startup, with CloseHandle() at shutdown. Then the Lock() and and Release() would just use the global mutex handle. BTW, since your code should not reach releaseLock() without having aquired a lock, releaseLock() should not wait(), but simply ReleaseMutex(). Back to top Bob Gonder Guest Posted: Sat Apr 28, 2007 11:49 pm    Post subject: Re: Need help with Mutex SubZero wrote: Quote: Ok. The original functions will be used with only TCriticalSection (single line). The reason for all the code is that it is needed for detecting the deadlock. I want to raise an exception so that MadExcept can show the stack! Hope you understand now. Sorry for the confusion. I cannot create the mutexes at startup because there are countless criticalsections. It must work within these two functions with named mutexes. You still need to call CreateMutex with false, not true. You still need to CloseHandle for every CreateMutex and OpenMutex. Suggested outline: whatever lockCritSec( whatever) { HANDLE M; BOOL opened = true; M = OpenMutex( flag, false, name); if( M == NULL ) { // create only if needed (first time) opened = false; M = CreateMutex( flag, false, name ); if( M == NULL ) {handle failure and return}; }; switch( WaitForSingleObject( M, 5000 ) ) {case WAIT_TIMEOUT: /*whatever */ default: break; }; if( opened ) CloseHandle( M ); } whatever releaseCritSec( whatever) { HANDLE M; M = OpenMutex( flag, false, name); if( M == NULL ) {handle failure and return}; // note that there is no wait() nor aquire at this point, // we should already own the mutex // and we only want to release it. ReleaseMutex( M ); CloseHandle( M ); } This way, at least you only have one dangling handle for each CritSec. Back to top Dennis Jones Guest Posted: Sun Apr 29, 2007 8:10 am    Post subject: Re: Need help with Mutex "SubZero" <g_ates (AT) hotmail (DOT) com> wrote in message news:4633887b (AT) newsgroups (DOT) borland.com... Quote: Ok. The original functions will be used with only TCriticalSection (single line). The reason for all the code is that it is needed for detecting the deadlock. I want to raise an exception so that MadExcept can show the stack! Hope you understand now. Sorry for the confusion. If you have madExcept, you don't need to throw an exception to get a stack trace. madExcept provides routines to get a stack trace at *any* point during program execution, not just when an exception is thrown. See the descriptions for StackTrace() and GetBugReport(). - Dennis Back to top Dennis Jones Guest Posted: Sun Apr 29, 2007 8:10 am    Post subject: Re: Need help with Mutex "Bob Gonder" <notbg (AT) notmindspring (DOT) invalid> wrote in message news:i717339g3u8esppurihllgq3aj0a5qelig (AT) 4ax (DOT) com... Quote: Dennis Jones wrote: Perhaps I am mistaken, but it looks to me like the problem is obvious...your comment says, "Create a mutex with no initial owner", but the code creates it owned (conflicts with the comment). Then you attempt to wait for the mutex, and 5 seconds later, due to timeout, the exception is thrown. The CreateMutex(true) returns with ownership. The wait() satisfies with pre-ownership to eliminate deadlock. Drawback is now the mutex is owned twice and should be released twice. I'm intrigued, but my brain is functioning on overload and a lack of sleep (only to get worse in the near future, I'm afraid). Could you please explain how this methodology works to eliminate deadlock? - Dennis Back to top SubZero Guest Posted: Sun Apr 29, 2007 1:18 pm    Post subject: Re: Need help with Mutex The problem is I am not sure when to get the stack trace! Because it deadlocks at some location we do not know... Thanks, SZ "Dennis Jones" <nospam (AT) nospam (DOT) com>, iletisinde þunu yazdý, news:4634170d$1 (AT) newsgroups (DOT) borland.com... Quote: "SubZero" <g_ates (AT) hotmail (DOT) com> wrote in message news:4633887b (AT) newsgroups (DOT) borland.com... Ok. The original functions will be used with only TCriticalSection (single line). The reason for all the code is that it is needed for detecting the deadlock. I want to raise an exception so that MadExcept can show the stack! Hope you understand now. Sorry for the confusion. If you have madExcept, you don't need to throw an exception to get a stack trace. madExcept provides routines to get a stack trace at *any* point during program execution, not just when an exception is thrown. See the descriptions for StackTrace() and GetBugReport(). - Dennis Back to top SubZero Guest Posted: Sun Apr 29, 2007 1:20 pm    Post subject: Re: Need help with Mutex Hello, I thought I could get rid of the mutexes with the below code: void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { for(int i = 0; i < 5000; ++i) { if(criticalSection->TryEnter()) return; ::Sleep(1); } throw Exception("BUG FOUND, CHECK CALL STACK!"); //criticalSection->Acquire(); }; but it never fires the exception yet it still deadlocks! ( Regards, SZ "SubZero" <g_ates (AT) hotmail (DOT) com>, iletisinde þunu yazdý, news:4633777f$1 (AT) newsgroups (DOT) borland.com... Quote: Hello, I used to have an application with the following functions for thread sync'in. void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { criticalSection->Acquire(); }; void inline __fastcall releaseCriticalSection(TCriticalSection *criticalSection) { criticalSection->Release(); }; To detect a deadlock, I implemented the below: void inline __fastcall lockCriticalSection(TCriticalSection *criticalSection) { HANDLE hMutex; // Create a mutex with no initial owner. hMutex = CreateMutex( NULL, // no security attributes true, // initially owned String((int)(void*)criticalSection).c_str()); // name of mutex DWORD dwWaitResult; // Request ownership of mutex. if(hMutex) { dwWaitResult = WaitForSingleObject( hMutex, // handle to mutex 5000L); // five-second time-out interval switch(dwWaitResult) { // Cannot get mutex ownership due to time-out. case WAIT_TIMEOUT: throw Exception("BUG FOUND, CHECK CALL STACK!"); // Got ownership of the abandoned mutex object. case WAIT_ABANDONED: break; } } criticalSection->Acquire(); }; void inline __fastcall releaseCriticalSection(TCriticalSection *criticalSection) { criticalSection->Release(); HANDLE hMutex; // Create a mutex with no initial owner. hMutex = OpenMutex( MUTEX_ALL_ACCESS, false, String((int)(void*)criticalSection).c_str() ); // name of mutex if(hMutex) { DWORD dwWaitResult; // Request ownership of mutex. dwWaitResult = WaitForSingleObject( hMutex, // handle to mutex 5000L); // five-second time-out interval switch(dwWaitResult) { // The thread got mutex ownership. case WAIT_OBJECT_0: // Release ownership of the mutex object. if(!ReleaseMutex(hMutex)) { // Deal with error. } break; // Cannot get mutex ownership due to time-out. case WAIT_TIMEOUT: break; // Got ownership of the abandoned mutex object. case WAIT_ABANDONED: break; } } }; which does not work for no obvious reason. It simply raises the exception when there is no reason! The Critical section variable is -1, 0 at that time! Please help! Best Regards, SZ Back to top Bob Gonder Guest Posted: Sun Apr 29, 2007 7:12 pm    Post subject: Re: Need help with Mutex Dennis Jones wrote: Quote: Could you please explain how this methodology works to eliminate deadlock? The wait() satisfies with pre-ownership to eliminate deadlock. Rephrase: pre-ownership satisfies the wait (eliminating deadlocking on self) If I own the mutex, and issue a wait, the wait returns with another ownership, it doesn't wait for me to release my original ownership. This allows you to re-use functions that lock/modify/release without affecting your own ownership. Back to top JD Guest Posted: Sun Apr 29, 2007 8:10 pm    Post subject: Re: Need help with Mutex Bob Gonder <notbg (AT) notmindspring (DOT) invalid> wrote: Quote: [...] which you don't use CloseHandle on, so are leaking resources From win32.hlp CreateMutex: Use the CloseHandle function to close the handle. The system closes the handle automatically when the process terminates. The mutex object is destroyed when its last handle has been closed. ~ JD Back to top Bob Gonder Guest Posted: Sun Apr 29, 2007 8:43 pm    Post subject: Re: Need help with Mutex JD wrote: Quote: Bob Gonder wrote: [...] which you don't use CloseHandle on, so are leaking resources From win32.hlp CreateMutex: And your point is? That the mutex is released if he closes the handle? My point was he was creating 2 handles for every lock/release, and never closing them. Won't take long to run out of handles. In my final example I showed how to create/leak only one handle per TCriticalSection object, no matter how many times it was used. Acceptable for testing purposes, as a few leaked handles shouldn't create it's own set of problems (like the run-away handles could). For on-going usage, I'd make the handle part of the class and have the destructor close it. But then, I prefere to write in a non-deadlocking manner in the first place. Back to top JD Guest Posted: Sun Apr 29, 2007 8:47 pm    Post subject: Re: Need help with Mutex Bob Gonder <notbg (AT) notmindspring (DOT) invalid> wrote: Quote: JD wrote: Bob Gonder wrote: [...] which you don't use CloseHandle on, so are leaking resources From win32.hlp CreateMutex: And your point is? That it's impossible to leak resources in this case because the OS cleans up. ~ JD Back to top Bob Gonder Guest Posted: Sun Apr 29, 2007 9:19 pm    Post subject: Re: Need help with Mutex JD wrote: Quote: And your point is? That it's impossible to leak resources in this case because the OS cleans up. When the program exits. Until then, the application can use up all available handles and then fail.. I think it would only take around 32 thousand lock cycles before failure. Adding another assured (but not intended) failure point in an attempt to track down an existing failure, seems a bit odd don't you think? 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