Date: Mon, 3 Jan 2005 23:20:26 GMT From: Giorgos Keramidas <keramida@freebsd.org> To: freebsd-doc@FreeBSD.org Subject: Re: docs/75571: man page for sx(9) is misleading Message-ID: <200501032320.j03NKQDR098896@freefall.freebsd.org>
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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 , %%%
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