Date: Mon, 9 Aug 2004 15:08:22 -0400 From: John Baldwin <jhb@FreeBSD.org> To: Alfred Perlstein <alfred@FreeBSD.org> Cc: Kris Kennaway <kris@obsecurity.org> Subject: Re: cvs commit: src/sys/i386/i386 pmap.c src/sys/kern subr_witness.c Message-ID: <200408091508.22530.jhb@FreeBSD.org> In-Reply-To: <20040808053321.GD57908@elvis.mu.org> References: <200408042031.i74KVKUf039025@repoman.freebsd.org> <20040808042703.GA64746@xor.obsecurity.org> <20040808053321.GD57908@elvis.mu.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sunday 08 August 2004 01:33 am, Alfred Perlstein wrote: > * Kris Kennaway <kris@obsecurity.org> [040807 21:28] wrote: > > On Wed, Aug 04, 2004 at 04:34:03PM -0400, John Baldwin wrote: > > > On Wednesday 04 August 2004 04:31 pm, John Baldwin wrote: > > > > jhb 2004-08-04 20:31:19 UTC > > > > > > > > FreeBSD src repository > > > > > > > > Modified files: > > > > sys/i386/i386 pmap.c > > > > sys/kern subr_witness.c > > > > Log: > > > > Remove a potential deadlock on i386 SMP by changing the lazypmap > > > > ipi and spin-wait code to use the same spin mutex (smp_tlb_mtx) as > > > > the TLB ipi and spin-wait code snippets so that you can't get into > > > > the situation of one CPU doing a TLB shootdown to another CPU that is > > > > doing a lazy pmap shootdown each of which are waiting on each other. > > > > With this change, only one of the CPUs would do an IPI and spin-wait > > > > at a time. > > > > > > Both this patch and the previous I have tested locally and also sent > > > out to current@ for testing. However, I received zero feedback (not > > > even useless feedback), so they may theoretically be risky. > > > > Isn't this the patch I tested for you and reported that it did not fix > > the problem? > > Y'know there's some existing research on these sort of low level > deadlocks, ie. how to do TLB shootdown without deadlock in: > > "UNIX Internals: The New Frontiers" Yes, and the failsafe algorithm is to halt all CPUs, do the change, then unhalt the CPUs forcing each one to do a TLB shootdown when it unfreezes. Unfortuantely, this has a very negative performance impact as the book also details. The current code was written by Peter who is not all that dumb, Alfred. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200408091508.22530.jhb>