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Subject: Re: [PATCH v3 0/6] Static calls
To:     Linus Torvalds <torvalds@linux-foundation.org>
CC:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Nadav Amit <namit@vmware.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
From:   hpa@zytor.com
Message-ID: <E2076BDA-FED4-449F-805F-F255755C78A2@zytor.com>
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On January 11, 2019 11:34:34 AM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Fri, Jan 11, 2019 at 11:24 AM <hpa@zytor.com> wrote:
>>
>> I still don't see why can't simply spin in the #BP handler until the
>patch is complete.
>
>So here's at least one problem:
>
>text_poke_bp()
>  text_poke(addr, &int3, sizeof(int3));
>   *interrupt*
>      interrupt has a static call
>        *BP*
>          poke_int3_handler
>             *BOOM*
>
>Note how at BOOM we cannot just spin (or return) to wait for the
>'int3' to be switched back. Becuase it never will. Because we are
>interrupting the thing that would do that switch-back.
>
>So we'd have to do the 'text_poke_bp()' sequence with interrupts
>disabled. Which we can't do right now at least, because part of that
>sequence involves that on_each_cpu(do_sync_core) thing, which needs
>interrupts enabled.
>
>See?
>
>Or am I missing something?
>
>            Linus

Ok, I was thinking far more about spinning with an IRET and letting the exception be delivered. Patching with interrupts disabled have other problems... 
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