Date: Tue, 21 Jun 2011 21:46:30 -0600 From: Warner Losh <imp@bsdimp.com> To: Julian Elischer <julian@freebsd.org> Cc: src-committers@freebsd.org, Alan Cox <alc@rice.edu>, Alan Cox <alc@freebsd.org>, svn-src-all@freebsd.org, Attilio Rao <attilio@freebsd.org>, "Bjoern A. Zeeb" <bz@freebsd.org>, Bruce Evans <brde@optusnet.com.au>, svn-src-head@freebsd.org Subject: Re: svn commit: r223307 - head/sys/vm Message-ID: <3B90907F-0CC4-4C2F-9386-6BF12E750F6C@bsdimp.com> In-Reply-To: <4E01612F.4080007@freebsd.org> References: <201106191913.p5JJDOqJ006272@svn.freebsd.org> <BBC34F79-FFA7-4A05-83B3-DE17E0AB14D0@FreeBSD.org> <20110622063258.D2275@besplex.bde.org> <BANLkTi=7WnYPQRwE4Hi472DuJz91d1sK=g@mail.gmail.com> <4E0128FF.6020804@rice.edu> <BEDB4F71-8151-4799-9272-8CE79CDA6C17@bsdimp.com> <4E01612F.4080007@freebsd.org>
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On Jun 21, 2011, at 9:27 PM, Julian Elischer wrote: > On 6/21/11 7:27 PM, Warner Losh wrote: >>=20 >>=20 >> On Jun 21, 2011, at 5:27 PM, Alan Cox wrote: >>=20 >>> On 06/21/2011 16:09, Attilio Rao wrote: >>>> 2011/6/21 Bruce Evans<brde@optusnet.com.au>: >>>>> On Tue, 21 Jun 2011, Bjoern A. Zeeb wrote: >>>>>=20 >>>>>> On Jun 19, 2011, at 7:13 PM, Alan Cox wrote: >>>>>>=20 >>>>>> Hi Alan, >>>>>>=20 >>>>>>> Author: alc >>>>>>> Date: Sun Jun 19 19:13:24 2011 >>>>>>> New Revision: 223307 >>>>>>> URL: http://svn.freebsd.org/changeset/base/223307 >>>>>>>=20 >>>>>>> Log: >>>>>>> Precisely document the synchronization rules for the page's = dirty field. >>>>>>> (Saying that the lock on the object that the page belongs to = must be >>>>>>> held >>>>>>> only represents one aspect of the rules.) >>>>>>>=20 >>>>>>> Eliminate the use of the page queues lock for atomically = performing >>>>>>> read- >>>>>>> modify-write operations on the dirty field when the underlying >>>>>>> architecture >>>>>>> supports atomic operations on char and short types. >>>>>>>=20 >>>>>>> Document the fact that 32KB pages aren't really supported. >>>>>> contrary to the tinderbox I'd like to point out that all mips = kernels >>>>>> built by universe are broken with a SVN HEAD from earlier today. = Could you >>>>>> please check and see if you can fix it? The errors I get are: >>>>>>=20 >>>>>> vm_page.o: In function `vm_page_clear_dirty': >>>>>> /sys/vm/vm_page.c:(.text+0x18d0): undefined reference to = `atomic_clear_8' >>>>>> /sys/vm/vm_page.c:(.text+0x18d0): relocation truncated to fit: = R_MIPS_26 >>>>>> against `atomic_clear_8' >>>>>> vm_page.o: In function `vm_page_set_validclean': >>>>>> /sys/vm/vm_page.c:(.text+0x38f0): undefined reference to = `atomic_clear_8' >>>>>> /sys/vm/vm_page.c:(.text+0x38f0): relocation truncated to fit: = R_MIPS_26 >>>>>> against `atomic_clear_8' >>>>> Atomic types shorter than int cannot be used in MI code, since = they might >>>>> not exist. Apparently they don't exist on mips. jake@ fixed all = their >>>>> old uses for sparc4 in ~Y2K. >>>> I'm sure they do, they exist in support.S though and may not have = the >>>> _8 form (they may just have the _char version). I may look at the = code >>>> again to be sure. >>>>=20 >>>=20 >>> It appears that while mips/include/atomic.h declares the existence = of atomic_clear_8, mips/mips/support.S doesn't implement it. In other = words, only support for int's and short's is currently implemented, not = char's: >>>=20 >>> # grep atomic_clear mips/mips/support.S >>> * atomic_clear_32(u_int32_t *a, u_int32_t b) >>> LEAF(atomic_clear_32) >>> END(atomic_clear_32) >>> * atomic_clear_16(u_int16_t *a, u_int16_t b) >>> LEAF(atomic_clear_16) >>> END(atomic_clear_16) >>=20 >> The current crop of atomic*16 and atomic*8 functions have the = restriction that the address must be 32-bit aligned (and it forces this = by aligning to 32-bits silently and then operates on the low 8 or 16 = bits in that word!) >>=20 >> I'm guessing that this is likely just wrong. Comments? >=20 > I'm guessing it depends on whether the hardware supports atomic = sub-wordline accesses. > (mind you it's getting really hard to work out what a wordline means = these days) Generally, they don't. ll and lld are for 4 and 8 byte values. You're = supposed to load the word or doubleword with the instruction, mask off = the byte you want to change, change it, and then write it back with the = sc or scd insturction. That will only succeed of none of the bytes in = that word could have changed (yes, weasel words usually meaning anything = in the cache line, but really can mean anything). I know that some cores do support special instructions to do this, but = they are with extensions to the ISA... Warner=
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