Date: Wed, 5 Jan 2005 15:49:58 -0500 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-doc@FreeBSD.org, Giorgos Keramidas <keramida@FreeBSD.org> Subject: Re: docs/75571: man page for sx(9) is misleading Message-ID: <200501051549.58949.jhb@FreeBSD.org> In-Reply-To: <200501032320.j03NKQDR098896@freefall.freebsd.org> References: <200501032320.j03NKQDR098896@freefall.freebsd.org>
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On Monday 03 January 2005 06:20 pm, Giorgos Keramidas wrote: > The following reply was made to PR docs/75571; it has been noted by GNATS. > > From: Giorgos Keramidas <keramida@freebsd.org> > To: John Baldwin <jhb@freebsd.org> > Cc: bug-followup@freebsd.org > Subject: Re: docs/75571: man page for sx(9) is misleading > Date: Tue, 4 Jan 2005 01:11:36 +0200 > > On 2004-12-29 13:34, John Baldwin <jhb@freebsd.org> wrote: > >On Wednesday 29 December 2004 03:40 am, Giorgos Keramidas wrote: > >>On 2004-12-28 13:55, Darren Reed <darrenr@FreeBSD.ORG> wrote: > >>> According to discussion on freebsd mailing lists, it is not possible > >>> to hold an sx lock when you want a mtx lock. This should be > >>> documented. > >> > >> As far as I can tell, by looking at kern_sx.c and sys/sx.h, this is > >> because the sx lock initialization uses an mtxpool for the mutex used > >> to serialize access to the internal sx lock data. [...] > > > > The reason is largely because they can be held across a sleep, e.g.: > > > > sx_xlock(&foo->sx); > > bar = malloc(sizeof(*bar), M_FOO, M_WAITOK); > > TAILQ_INSERT_TAIL(&foo->head, bar, link); > > sx_xunlock(&foo->sx); > > > > This is intentional and that is what should be documented. Basically, > > it needs a paragraph to the effect of: > > > > .Pp > > An > > .Nm > > lock may not be acquired while holding a mutex. > > Since threads are allowed to sleep while holding an > > .NM > > lock, > > a thread that acquired a mutex and then blocked on an > > .Nm > > lock would end up sleeping while holding a mutex which is not allowed. > > Nice :-) > > Thanks for putting this in words. Should I commit this? > > %%% > Index: sx.9 > =================================================================== > RCS file: /home/ncvs/src/share/man/man9/sx.9,v > retrieving revision 1.29 > diff -u -5 -r1.29 sx.9 > --- sx.9 11 Jul 2004 16:08:25 -0000 1.29 > +++ sx.9 3 Jan 2005 23:08:40 -0000 > @@ -194,10 +194,19 @@ > attempting to do so will result in deadlock. > .Sh CONTEXT > A thread may hold a shared or exclusive lock on an > .Nm > lock while sleeping. > +As a result, an > +.Nm > +lock may not be acquired while holding a mutex. > +Since threads are allowed to sleep while holding an > +.Nm > +lock, > +a thread that acquired a mutex and then blocked on an > +.Nm > +lock would end up sleeping while holding a mutex which is not allowed. > .Sh SEE ALSO > .Xr condvar 9 , > .Xr mtx_pool 9 , > .Xr mutex 9 , > .Xr panic 9 , > %%% Hmm, that's a good place to put that, here's a minor wording tweak: As a result, an .Nm lock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an .Nm lock while another thread blocked on the same .Nm lock after acquiring a mutex, then the second thread would effectively end up sleeping while holding a mutex. Eesh, any way you say it it ends up being a mouthful. :-P -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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