Date: Mon, 16 Aug 2010 13:43:03 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: David Xu <davidxu@freebsd.org> Cc: freebsd-threads@freebsd.org Subject: Re: PTHREAD_CANCEL_DEFERRED Message-ID: <20100816104303.GP2396@deviant.kiev.zoral.com.ua> In-Reply-To: <4C696A96.7020709@freebsd.org> References: <4C642E9B.8000300@freebsd.org> <20100812093353.GS2396@deviant.kiev.zoral.com.ua> <4C650D0F.9060905@freebsd.org> <4C650F27.1000305@freebsd.org> <20100813141402.GW2396@deviant.kiev.zoral.com.ua> <4C65E0FE.2030803@freebsd.org> <20100814144715.GB2396@deviant.kiev.zoral.com.ua> <4C6926D0.2020909@freebsd.org> <20100816082022.GO2396@deviant.kiev.zoral.com.ua> <4C696A96.7020709@freebsd.org>
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--eUGqGfkt69yEwRle Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Aug 16, 2010 at 04:43:02PM +0000, David Xu wrote: > Kostik Belousov wrote: > >On Mon, Aug 16, 2010 at 11:53:52AM +0000, David Xu wrote: > >>Kostik Belousov wrote: > >> > >>>Missed this, thank you for pointing it out. Updated patch is at > >>>http://people.freebsd.org/~kib//misc/cancel_defer.2.patch > >>I found SIGCANCEL is masked by > >>thr_cancel_deferred(THR_CANCEL_DEFERRED_ENABLE), issignal() does not > >>return the masked signal, so how a cancellation point syscall can be > >>interrupted by SIGCANCEL ? I think if a thread being canceled calls > >>msleep(PCATCH), it should find the signal and return EINTR. > >> > >Yes, for EINTR and ERESTART case, the thread should be canceled. > >Please look at the check_cancel() helper that is called at the syscall > >entry and before return. If the check_cancel() decided that the syscall > >is cancellation point and the thread in the deferred cancel mode, and > >EINTR or ERESTART is supplied as error code, then SIGCANCEL is removed > >from the thread signal mask. It is restored in the mask by ast(). >=20 > I saw your patch has following lines, on syscall entry, if the > check_cancel finds that the syscall is cancellation point and > SIGCANCEL exists, it returns non-zero value, then the real syscall > body at line 319 of subr_trap.c is not executed and prematurely returns. > This is not what I want,as you said the syscall's implementation should > always be executed, and if the thread will be blocked, then it should be > interrupted by existing SIGCANCEL via issignal() which is called by > sleepqueue routines. I think the current patch also has potential=20 No, I do not think what you describe is completely right behaviour. As I understand SUSv4, the deferred cancel behaviour should be as following: - when we reach cancellation point with cancel already pending, the syscall should not be executed at all, cancel happens and cleanup handlers executed before thread exiting. - when we reach cancellation point without cancel pending, and syscall goes to sleep, and cancellation request arrives during the sleep, again the syscall should be aborted, and cancellation happens. So I check for the cancellation at two points: one is the syscall entry, to catch already pending cancellation request. Second is on the syscall exit path, where cancellation is only checked in the case the sleep was aborted. > performance problem, the check_cancel uses PROC_LOCK, which might be a=20 > performance hit. Might be. Lets first make sure that the semantic is right. >=20 > @@ -300,6 +332,9 @@ syscallenter(struct thread *td, struct syscall_args *= sa) > if (error !=3D 0) > goto retval; > } > + error =3D check_cancel(td, sa->callp, EINTR); > + if (error !=3D 0) > + goto retval; > error =3D syscall_thread_enter(td, sa->callp); > if (error !=3D 0) > goto retval; >=20 --eUGqGfkt69yEwRle Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (FreeBSD) iEYEARECAAYFAkxpFjcACgkQC3+MBN1Mb4hCawCfclQJULuAgsYI/SKyNkpbx/VK JrYAmgO98aEv0Jydq/ETSdd3IDL0wAe/ =59mj -----END PGP SIGNATURE----- --eUGqGfkt69yEwRle--
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