Date: Wed, 29 Dec 2004 13:34:17 -0500 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-doc@FreeBSD.org, Giorgos Keramidas <keramida@ceid.upatras.gr> Subject: Re: docs/75571: man page for sx(9) is misleading Message-ID: <200412291334.17967.jhb@FreeBSD.org> In-Reply-To: <200412290840.iBT8eQuf017631@freefall.freebsd.org> References: <200412290840.iBT8eQuf017631@freefall.freebsd.org>
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On Wednesday 29 December 2004 03:40 am, Giorgos Keramidas wrote: > The following reply was made to PR docs/75571; it has been noted by GNATS. > > From: Giorgos Keramidas <keramida@ceid.upatras.gr> > To: bug-followup@freebsd.org > Cc: > Subject: Re: docs/75571: man page for sx(9) is misleading > Date: Wed, 29 Dec 2004 10:37:13 +0200 > > 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. > > Leaf locks may be used in operations that msleep() but there can be only > one of them in each lock path and no other lock can be obtained after > them. > > This is sort of implied by the SEE ALSO reference of mtx_pool(9), but we > should probably state it explicitly in CONTEXT. 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. > %%% > Index: sx.9 > =================================================================== > RCS file: /home/ncvs/src/share/man/man9/sx.9,v > retrieving revision 1.29 > diff -u -r1.29 sx.9 > --- sx.9 11 Jul 2004 16:08:25 -0000 1.29 > +++ sx.9 28 Dec 2004 23:28:22 -0000 > @@ -196,6 +196,11 @@ > A thread may hold a shared or exclusive lock on an > .Nm > lock while sleeping. > +The > +.Nm > +locks are implemented using > +.Xr mtxpool 9 > +shared leaf locks, so they should always be the last lock obtained. > .Sh SEE ALSO > .Xr condvar 9 , > .Xr mtx_pool 9 , > %%% > _______________________________________________ > freebsd-doc@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-doc > To unsubscribe, send any mail to "freebsd-doc-unsubscribe@freebsd.org" -- 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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