Date: Sun, 02 Sep 2007 23:23:38 -0400 From: Joe Marcus Clarke <marcus@FreeBSD.org> To: Bruce Evans <brde@optusnet.com.au> Cc: Jilles Tjoelker <jilles@stack.nl>, freebsd-arch@FreeBSD.org Subject: Re: Understanding interrupted system calls Message-ID: <1188789818.97969.9.camel@shumai.marcuscom.com> In-Reply-To: <20070902131910.H46281@delplex.bde.org> References: <1188600721.1255.11.camel@shumai.marcuscom.com> <20070901112600.GA33832@stack.nl> <1188660782.41727.5.camel@shumai.marcuscom.com> <20070901224025.GA97796@stack.nl> <20070902131910.H46281@delplex.bde.org>
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[-- Attachment #1 --] On Sun, 2007-09-02 at 14:17 +1000, Bruce Evans wrote: > On Sun, 2 Sep 2007, Jilles Tjoelker wrote: > > > On Sat, Sep 01, 2007 at 11:33:02AM -0400, Joe Marcus Clarke wrote: > >> However, I'm curious as to my other point in this thread. Why should > >> one need to re-register the default signal handlers to get a syscall to > >> return EINTR? Or should ERESTART be caught and turned into EINTR then > >> return to the caller (as in kern_connect())? > > > > It is intended that most blocking system calls are not interrupted by > > signals. This saves the programmer some checks on EINTR. > > Yes, I think this is just the historical default for BSD. Not > restarting syscalls is very unusual in BSD, and this was implemented > by defaulting ps_sigintr to all unset (?). Before sigaction(2) existed, > signal(3) was probably signal(2), and most programs used only signal() > and got the default. siginterrupt(3) was more probably siginterrupt(2), > and the few programs that understand this stuff and want to change the > default had to use it to get EINTR. Now with sigaction(2), the default > doesn't apply to userland, since setting up a signal catcher requires > using sigaction(2) which sets the ps_sigintr bit to the inverse of the > SA_RESTART bit in the sigaction data. However, the default might still > affect operation in the kernel for programs that never call sigaction(). > I think it shouldn't have any effect except to return to near the > syscall entry point to decide what to do. Lower levels should see > ERESTART and return to near the syscall entry point. Similarly if > sigaction() is used to change the default. Thanks, Bruce and Jilles for the explanation. It is much clearer now. > > > Some system calls, e.g. connect(), read/write/etc that have already > > committed some data and under BSD also select/poll/kqueue do not restart > > and always return EINTR or partial success. In the kernel code, this > > appears as changing ERESTART to EINTR. > > This translation happens at a high level, but the translation of ps_sigintr > happens at the [*]sleep() level. So ps_sigintr has a significant affect > at a low level, but only (?) to select between ERESTART and EINTR. > > >From Jilles' previous reply: > > >>> The problem seems to be the following code in > >>> src/sys/dev/syscons/syscons.c, in case VT_WAITACTIVE in scioctl(): > >>> > >>> while ((error=tsleep(&scp->smode, PZERO|PCATCH, > >>> "waitvt", 0)) == ERESTART) ; > >>> > >>> If a signal is caught and system call restart is enabled for that > >>> signal, this makes it spin in a tight loop, waiting in vain for the > >>> signal to go away. The idea of ERESTART is that the syscall function > >>> returns it and then the signal handler is entered. If and when the > >>> signal handler returns, it will return to the system call instruction, > >>> restarting it (perhaps this is optimized to avoid the switch to userland > >>> and back). With EINTR, the signal handler would return to directly > >>> after the system call instruction. > >>> > >>> The fixed version would then be > >>> > >>> error = tsleep(&scp->smode, PZERO|PCATCH, "waitvt", 0); > > I think this is right. The kernel should never loop on ERESTART like this. > Please fix the remaining style bug in it (missing spaces around binary > operator). I think these patches do what you and Jilles suggested. As Jilles said, the pcvt patch only applies to -STABLE as this driver was removed in -CURRENT. Thanks again, guys, for helping me out with this. http://www.marcuscom.com/downloads/syscons.c.diff http://www.marcuscom.com/downloads/pcvt_ext.c.diff Joe -- Joe Marcus Clarke FreeBSD GNOME Team :: gnome@FreeBSD.org FreeNode / #freebsd-gnome http://www.FreeBSD.org/gnome [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (FreeBSD) iD8DBQBG2346b2iPiv4Uz4cRAsJwAKCJacuQMP+LUCxYbiWQed5EV6gJAQCfaTFP cONVTPYttoSv1A3bhmrPGtY= =d8MY -----END PGP SIGNATURE-----home | help
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