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Date: Fri, 5 Apr 2019 17:30:27 +0100 From: Will Deacon <will.deacon@....com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: linux-arch <linux-arch@...r.kernel.org>, Linux List Kernel Mailing <linux-kernel@...r.kernel.org>, "Paul E. McKenney" <paulmck@...ux.ibm.com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Michael Ellerman <mpe@...erman.id.au>, Arnd Bergmann <arnd@...db.de>, Peter Zijlstra <peterz@...radead.org>, Andrea Parri <andrea.parri@...rulasolutions.com>, Palmer Dabbelt <palmer@...ive.com>, Daniel Lustig <dlustig@...dia.com>, David Howells <dhowells@...hat.com>, Alan Stern <stern@...land.harvard.edu>, "Maciej W. Rozycki" <macro@...ux-mips.org>, Paul Burton <paul.burton@...s.com>, Ingo Molnar <mingo@...nel.org>, Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>, Mikulas Patocka <mpatocka@...hat.com>, Akira Yokosawa <akiyks@...il.com>, Luis Chamberlain <mcgrof@...nel.org>, Nicholas Piggin <npiggin@...il.com> Subject: Re: [PATCH v2 00/21] Remove Mysterious Macro Intended to Obscure Weird Behaviours (mmiowb()) On Fri, Apr 05, 2019 at 06:15:12AM -1000, Linus Torvalds wrote: > On Fri, Apr 5, 2019 at 6:09 AM Will Deacon <will.deacon@....com> wrote: > > > > > > Or did I miss something? I think the ia64() mb/rmb/wmb stuff only > > > works on normal memory on ia64. > > > > I was worried about RISC-V, but actually their wmb() is "fence ow,ow" > > which I think is stronger than their mmiowb() "fence o,w" implementation. > > Also with smp_store_release -> smp_load_acquire kind of ordering? Hmm, to be honest, I'm not convinced that smp_load_acquire() is ordered wrt subsequent I/O on RISC-V anyway, so in the pattern of: CPU 0: writel(1, dev); wmb(); smp_store_release(&x, 1); CPU 1: if (smp_load_acquire(&x) == 1) writel(2, dev) then I think it's actually the control dependency in CPU 1 that provides the expected ordering. That's probably quite fragile. > Again, this is not at all a NAK - I think we should do this - just > perhaps a request to add a note to the commit and make people aware of > the issue. Right, I'll do that. Will
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