Date: Sat, 7 Oct 2017 13:54:05 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: kostikbel@gmail.com Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r324313 - head/sys/amd64/amd64 Message-ID: <201710072054.v97Ks5nX062548@gw.catspoiler.org> In-Reply-To: <20171007201142.GE95911@kib.kiev.ua>
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On 7 Oct, Konstantin Belousov wrote: > On Sat, Oct 07, 2017 at 01:04:09PM -0700, Don Lewis wrote: >> On 5 Oct, Konstantin Belousov wrote: >> > Author: kib >> > Date: Thu Oct 5 12:50:03 2017 >> > New Revision: 324313 >> > URL: https://svnweb.freebsd.org/changeset/base/324313 >> > >> > Log: >> > Avoid a race betweem freeing LDT and context switches. >> > >> > cpu_switch.S uses curproc->p_md.md_ldt value as the flag indicating >> > presence of the process LDT. The flag is checked and then ldt segment >> > descriptor is copied into the CPU' GDT slot. >> > >> > Disallow context switches around clearing of the curproc LDT state by >> > performing the cleanup in critical section. Ensure that the md_ldt >> > flag is cleared before md_ldt_sd descriptor content is destroyed by >> > inserting fence between the operations. >> > >> > We depend on the x86 memory model strong ordering guarantees, in >> > particular, that cpu_switch.S observes the writes to md_ldt and >> > md_ldt_sd in the expected order. >> >> I don't know which of this series of commits is responsible, but I think >> that it fixed the build of lang/ghc on Ryzen. >> >> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221029#c102 > > Does ghc use LDT on amd64 ? This sounds unbelievable. I have no idea, but ghc would reliably fail to build on my Ryzen machine up through r323398, and at r324367 it seems to reliably build.
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