Re: mutex held in a thread which is cancelled stays busy

Hi,

On Wed, 7 Aug 2019 06:07:25 -0400
Daniel Eischen <[email protected]> wrote:

> > On Aug 7, 2019, at 5:20 AM, Konstantin Belousov
> > <[email protected]> wrote: 
> >> On Wed, Aug 07, 2019 at 04:37:57PM +0800, Erich Dollansky wrote:
> >> Hi,
> >> 
> >> On Wed, 7 Aug 2019 10:10:02 +0300
> >> Konstantin Belousov <[email protected]> wrote:
> >>   
> >>>> On Tue, Aug 06, 2019 at 08:58:30PM -0400, Daniel Eischen wrote:
> >>>>   
> >>>>> On Aug 6, 2019, at 4:54 AM, Erich Dollansky
> >>>>> <[email protected]> wrote:
> >>>>> 
> >>>>> Hi,
> >>>>> 
> >>>>> for testing purpose, I did the following.
> >>>>> 
> >>>>> Start a thread, initialise a mutex in a global variable, lock
> >>>>> the mutex and wait in that thread.
> >>>>> 
> >>>>> Wait in the main program until above's thread waits and cancel
> >>>>> it.
> >>>>> 
> >>>>> Clean up behind the cancelled thread but leave intentional the
> >>>>> mutex locked.
> >>>>> 
> >>>>> I would have expected now to get an error like 'EOWNERDEAD'
> >>>>> doing operations with that mutex. But I get 'EBUSY' as the
> >>>>> error.    
> >>>> 
> >>>> Are you initializing the mutex as a robust mutex, via
> >>>> pthread_mutexattr_setrobust()?  Are you using _lock() or
> >>>> _trylock()?   
> >>> Robust mutexes only have special properties on the process
> >>> termination. They behave same as the normal mutexes if the owning
> >>> thread is terminated.
> >>>   
> >> man says:
> >> 
> >> [EOWNERDEAD]  The argument mutex points to a robust mutex and the
> >> previous owning thread terminated while holding the mutex lock.  
> > 
> > So what ?  It describes the case when error can be returned, but it
> > is not required to do so.  POSIX wording is the following:
> > 
> > If mutex is a robust mutex and the process containing the owning
> > thread terminated while holding the mutex lock, a call to
> > pthread_mutex_lock() shall return the error value [EOWNERDEAD]. If
> > mutex is a robust mutex and the owning thread terminated while
> > holding the mutex lock, a call to pthread_mutex_lock( ) may return
> > the error value [EOWNERDEAD] even if the process in which the
> > owning thread resides has not terminated.
> > 
> > Note the difference between shall and may.  We only process robust
> > list on the process termination.  If the process is still alive,
> > but the thread terminated, it can only happen because the process
> > code asked for the thread termination explicitly, and then the code
> > should be able to keep its own state.  On really fatal conditions,
> > like unhandled signals, kernel terminates the process, not a
> > thread.  
> 
> But pthread_mutex_lock() should not return EBUSY; that is only for
> _trylock().  It seems to me _lock() should either return EOWNERDEAD
> or EDEADLK, or it just blocks indefinitely.
> 
> Erich, are you getting EBUSY for pthread_mutex_lock() or is that only
> for pthread_mutex_trylock()?
> 
EBUSY is only returned when I call 'pthread_mutex_trylock'. The other
one just hangs.

Give me a bit if time and I will send you then a test program which is
extracted from my test environment. Not that the error stems from
very own test environment.

Erich
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