Date: Wed, 1 Feb 2017 08:39:05 -0800 From: David Chisnall <theraven@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Alexander Kabaev <kabaev@gmail.com>, Alexander Kabaev <kan@FreeBSD.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r313040 - head/sys/mips/include Message-ID: <191141D2-4665-4E24-AA4F-A34127C15953@FreeBSD.org> In-Reply-To: <20170201161354.GF2092@kib.kiev.ua> References: <201702010500.v1150YLR075051@repo.freebsd.org> <20170201141721.GB2092@kib.kiev.ua> <20170201103842.1ad0989f@kan> <20170201161354.GF2092@kib.kiev.ua>
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--Apple-Mail=_6EFDB1DE-38E6-4B6F-A040-DCE2AAF02419 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii On 1 Feb 2017, at 08:13, Konstantin Belousov <kostikbel@gmail.com> = wrote: >=20 > On Wed, Feb 01, 2017 at 10:38:42AM -0500, Alexander Kabaev wrote: >> On Wed, 1 Feb 2017 16:17:21 +0200 >> Konstantin Belousov <kostikbel@gmail.com> wrote: >>=20 >>> Please do not retry on sc failure, return the error to upper layer. >>> See also r313007 and preceeding discussion after r312973. >>=20 >> There was not much a discussion there, do you mind expanding a bit on >> why one behavior is more desired than other? I am not against the >> change, but I need to understand the reasoning behind it better. = Since >> atomic_cmpset retries too, it will have to be adjusted as well. >=20 > atomic_cmpset() cannot avoid retry on the ll/sc architectures, because > sc might fail even if the old and the new values are same. One of the > points of the fcmpset API design is to avoid nested loops: this is a > microoptimization to put less pressure on the CPUs frontend. The = caller > of (f)cmpset must check for failure anyway, so not doing this inside = the > function reduces number of branches. Less branches makes code shorter, > and reduces utilization of some CPU resources, like branch predictor > state. C[++]11 addresses this by having a weak and a strong variant of compare = and exchange. The strong version may only fail if the comparison fails, = we weak version is permitted to fail spuriously. Given that most uses = of compare and exchange use a loop, and most ll/sc architectures = guarantee forward process after a few attempts, you almost always want = to use the weak version. The weak version also has the advantage that the compiler is free to = fold the initial load into the load linked, as long as the target = architecture would permit it, so you end up with more idiomatic ll, op, = sc, branch sequences, rather than l, op, ll, branch, sc, branch = sequences. 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