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Date: Tue, 6 Mar 2018 14:00:59 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Will Deacon <will.deacon@....com>, Luc Maranget <luc.maranget@...ia.fr>, Daniel Lustig <dlustig@...dia.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Andrea Parri <parri.andrea@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Palmer Dabbelt <palmer@...ive.com>, Albert Ou <albert@...ive.com>, Alan Stern <stern@...land.harvard.edu>, Boqun Feng <boqun.feng@...il.com>, Nicholas Piggin <npiggin@...il.com>, David Howells <dhowells@...hat.com>, Jade Alglave <j.alglave@....ac.uk>, Akira Yokosawa <akiyks@...il.com>, Ingo Molnar <mingo@...nel.org>, linux-riscv@...ts.infradead.org Subject: Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock() On Mon, Feb 26, 2018 at 09:00:43AM -0800, Linus Torvalds wrote: > On Mon, Feb 26, 2018 at 8:24 AM, Will Deacon <will.deacon@....com> wrote: > > > > Strictly speaking, that's not what we've got implemented on arm64: only > > the read part of the RmW has Acquire semantics, but there is a total > > order on the lock/unlock operations for the lock. > > Hmm. > > I thought we had exactly that bug on some architecture with the queued > spinlocks, and people decided it was wrong. So ARM64 and Power have the acquire-on-load only thing, but qspinlock has it per construction on anything that allowes reordering stores. Given that unlock/lock are ordered, which covers about 99% of the users out there, and fixing the issue would make things significantly slower on the weak architectures we let it be. But yes, its a pesky detail.
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