Date: Sat, 24 Jan 2015 12:33:07 -0600 From: Alan Cox <alc@rice.edu> To: Konstantin Belousov <kostikbel@gmail.com>, 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: <54C3E563.4070903@rice.edu> In-Reply-To: <20150124154240.GV42409@kib.kiev.ua> References: <201501241251.t0OCpGa8053192@svn.freebsd.org> <1422111397.1038.53.camel@freebsd.org> <20150124154240.GV42409@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
On 01/24/2015 09:42, 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. > > While we're here, it's worth noting that the arm version of /dev/mem is not functionally equivalent to that of amd64 or i386. Arm disallows access to non-DRAM addresses through /dev/mem.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?54C3E563.4070903>