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Date: Fri, 22 Feb 2019 13:55:20 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Will Deacon <will.deacon@....com> 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> Subject: Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking On Fri, Feb 22, 2019 at 1:49 PM Linus Torvalds <torvalds@...ux-foundation.org> wrote: > > The case we want to go fast is the spin-lock and unlock case, not the > "set pending" case. > > And the way you implemented this, it's exactly the wrong way around. Oh, one more comment: couldn't we make that mmiowb flag be right next to the preemption count? Because that's the common case anyway, where a spinlock increments the preemption count too. If we put the mmiowb state in the same cacheline, we don't cause extra cache effects, which is what really matters, I guess. I realize this is somewhat inconvenient, because some architectures put preempt count in the thread structure, and others do it as a percpu variable. But maybe the architecture could just declare where the mmiowb state is? Linus
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