Date: Sat, 24 Jan 2015 17:42:40 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Ian Lepore <ian@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r277643 - in head/sys: arm/arm dev/mem i386/i386 mips/mips sparc64/sparc64 Message-ID: <20150124154240.GV42409@kib.kiev.ua> In-Reply-To: <1422111397.1038.53.camel@freebsd.org> References: <201501241251.t0OCpGa8053192@svn.freebsd.org> <1422111397.1038.53.camel@freebsd.org>
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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.
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