Date: Wed, 7 Aug 2019 13:26:31 -0400 From: Daniel Eischen <deischen@freebsd.org> To: Erich Dollansky <freebsd.ed.lists@sumeritec.com> Cc: freebsd-questions@freebsd.org, freebsd-threads@freebsd.org Subject: Re: mutex held in a thread which is cancelled stays busy Message-ID: <BC58A1D4-C77A-49F4-B6D6-F85A8C13832E@freebsd.org> In-Reply-To: <20190807182548.1a8e00dd.freebsd.ed.lists@sumeritec.com> References: <20190806165429.14bc4052.freebsd.ed.lists@sumeritec.com> <1FC05CEB-982F-484F-9E41-5A74FF564494@freebsd.org> <20190807071002.GF2731@kib.kiev.ua> <20190807163757.2b5d52fa.freebsd.ed.lists@sumeritec.com> <20190807092035.GG2731@kib.kiev.ua> <AFA42FF8-C49F-495F-BD4A-F9FBB9301F5E@freebsd.org> <20190807182548.1a8e00dd.freebsd.ed.lists@sumeritec.com>
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> On Aug 7, 2019, at 6:25 AM, Erich Dollansky <freebsd.ed.lists@sumeritec.co= m> wrote: >=20 > Hi, >=20 > On Wed, 7 Aug 2019 06:07:25 -0400 > Daniel Eischen <deischen@freebsd.org> wrote: >=20 >>> On Aug 7, 2019, at 5:20 AM, Konstantin Belousov >>> <kostikbel@gmail.com> wrote:=20 >>>> On Wed, Aug 07, 2019 at 04:37:57PM +0800, Erich Dollansky wrote: >>>> Hi, >>>>=20 >>>> On Wed, 7 Aug 2019 10:10:02 +0300 >>>> Konstantin Belousov <kostikbel@gmail.com> wrote: >>>>=20 >>>>>>> On Tue, Aug 06, 2019 at 08:58:30PM -0400, Daniel Eischen wrote: >>>>>>>=20 >>>>>>> On Aug 6, 2019, at 4:54 AM, Erich Dollansky >>>>>>> <freebsd.ed.lists@sumeritec.com> wrote: >>>>>>>=20 >>>>>>> Hi, >>>>>>>=20 >>>>>>> for testing purpose, I did the following. >>>>>>>=20 >>>>>>> Start a thread, initialise a mutex in a global variable, lock >>>>>>> the mutex and wait in that thread. >>>>>>>=20 >>>>>>> Wait in the main program until above's thread waits and cancel >>>>>>> it. >>>>>>>=20 >>>>>>> Clean up behind the cancelled thread but leave intentional the >>>>>>> mutex locked. >>>>>>>=20 >>>>>>> I would have expected now to get an error like 'EOWNERDEAD' >>>>>>> doing operations with that mutex. But I get 'EBUSY' as the >>>>>>> error. =20 >>>>>>=20 >>>>>> Are you initializing the mutex as a robust mutex, via >>>>>> pthread_mutexattr_setrobust()? Are you using _lock() or >>>>>> _trylock()? =20 >>>>> Robust mutexes only have special properties on the process >>>>> termination. They behave same as the normal mutexes if the owning >>>>> thread is terminated. >>>>>=20 >>>> man says: >>>>=20 >>>> [EOWNERDEAD] The argument mutex points to a robust mutex and the >>>> previous owning thread terminated while holding the mutex lock. =20 >>>=20 >>> So what ? It describes the case when error can be returned, but it >>> is not required to do so. POSIX wording is the following: >>>=20 >>> 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. >>>=20 >>> 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. =20 >>=20 >> 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. >>=20 >> Erich, are you getting EBUSY for pthread_mutex_lock() or is that only >> for pthread_mutex_trylock()? >>=20 > EBUSY is only returned when I call 'pthread_mutex_trylock'. The other > one just hangs. In this case, I think FreeBSD is behaving correctly. I think perhaps the on= ly problem is that the man page isn't reflecting the POSIX wording. -- DE=
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