Date: Sun, 25 Jan 2015 19:20:36 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Warner Losh <imp@bsdimp.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers <src-committers@freebsd.org>, Ian Lepore <ian@freebsd.org> Subject: Re: svn commit: r277643 - in head/sys: arm/arm dev/mem i386/i386 mips/mips sparc64/sparc64 Message-ID: <20150125172036.GB42409@kib.kiev.ua> In-Reply-To: <DB07559A-5EE9-4495-ABBC-19D6E45B99EF@bsdimp.com> References: <201501241251.t0OCpGa8053192@svn.freebsd.org> <1422111397.1038.53.camel@freebsd.org> <20150124154240.GV42409@kib.kiev.ua> <20150124155117.GW42409@kib.kiev.ua> <DB07559A-5EE9-4495-ABBC-19D6E45B99EF@bsdimp.com>
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On Sun, Jan 25, 2015 at 10:07:00AM -0700, Warner Losh wrote: > > > On Jan 24, 2015, at 8:51 AM, Konstantin Belousov <kostikbel@gmail.com> wrote: > > > > On Sat, Jan 24, 2015 at 05:42:40PM +0200, Konstantin Belousov wrote: > >> On Sat, Jan 24, 2015 at 07:56:37AM -0700, Ian Lepore wrote: > >>> On Sat, 2015-01-24 at 12:51 +0000, Konstantin Belousov wrote: > >>>> Author: kib > >>>> Date: Sat Jan 24 12:51:15 2015 > >>>> New Revision: 277643 > >>>> URL: https://svnweb.freebsd.org/changeset/base/277643 > >>>> > >>>> Log: > >>>> Remove Giant from /dev/mem and /dev/kmem. It is definitely not needed > >>>> for i386, and from the code inspection, nothing in the > >>>> arm/mips/sparc64 implementations depends on it. > >>>> > >>> > >>> I'm not sure I agree with that. On arm the memrw() implementation uses > >>> a single statically-allocated page of kva space into which it maps each > >>> physical page in turn in the main loop. What prevents preemption or > >>> multicore access to /dev/mem from trying to use that single page for > >>> multiple operations at once? > >> > >> I see, thank you for noting this. > >> > >> But, I do not think that Giant is a solution for the problem. uiomove() > >> call accesses userspace, which may fault and cause sleep. If the > >> thread sleeps, the Giant is automatically dropped, so there is no real > >> protection. > >> > >> I think dump exclusive sx around whole memrw() should be enough. > >> > >> I can revert the commit for now, or I can leave it as is while > >> writing the patch with sx and waiting for somebody review. What > >> would you prefer ? > >> > >> P.S. mips uses uiomove_fromphys(), avoiding transient mapping, > >> and sparc allocates KVA when needed. > > > > Like this. > > So why a sx lock and not a mutex? I explained this above. uiomove() needs to sleep on fault.
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