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Date: Sun, 26 Aug 2018 11:09:58 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: TLB flushes on fixmap changes
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To: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>, Kees Cook <keescook@chromium.org>, Nadav Amit <nadav.amit@gmail.com>, Linus Torvalds <torvalds@linux-foundation.org>, Paolo Bonzini <pbonzini@redhat.com>, Jiri Kosina <jkosina@suse.cz>, Will Deacon <will.deacon@arm.com>, Benjamin Herrenschmidt <benh@au1.ibm.com>, Nick Piggin <npiggin@gmail.com>, the arch/x86 maintainers <x86@kernel.org>, Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>, Jann Horn <jannh@google.com>, Adin Scannell <ascannell@google.com>, Dave Hansen <dave.hansen@intel.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, David Miller <davem@davemloft.net>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Michael Ellerman <mpe@ellerman.id.au>

On Sat, Aug 25, 2018 at 09:21:22PM -0700, Andy Lutomirski wrote:
> I just re-read text_poke().  It's, um, horrible.  Not only is the
> implementation overcomplicated and probably buggy, but it's SLOOOOOW.
> It's totally the wrong API -- poking one instruction at a time
> basically can't be efficient on x86.  The API should either poke lots
> of instructions at once or should be text_poke_begin(); ...;
> text_poke_end();.

I don't think anybody ever cared about performance here. Only
correctness. That whole text_poke_bp() thing is entirely tricky.

FWIW, before text_poke_bp(), text_poke() would only be used from
stop_machine, so all the other CPUs would be stuck busy-waiting with
IRQs disabled. These days, yeah, that's lots more dodgy, but yes
text_mutex should be serializing all that.

And on that, I so hate comments like: "must be called under foo_mutex",
we have lockdep_assert_held() for that.