From mboxrd@z Thu Jan  1 00:00:00 1970
From: nstange@suse.de (Nicolai Stange)
Date: Sun, 28 Apr 2019 23:22:54 +0200
Subject: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member
In-Reply-To: <20190428135143.09d35bb6@oasis.local.home> (Steven Rostedt's
 message of "Sun, 28 Apr 2019 13:51:43 -0400")
References: <20190427100639.15074-1-nstange@suse.de>
 <20190427100639.15074-2-nstange@suse.de>
 <F6268634-96D3-4267-B739-6D375FAB26A8@amacapital.net>
 <20190428135143.09d35bb6@oasis.local.home>
Message-ID: <87k1fdygcx.fsf@suse.de>
Content-Type: text/plain; charset="UTF-8"
Message-ID: <20190428212254.eYidGDvN3M7xXfssHbHedJp2CcOcBnL7R1OiS5jX2vQ@z>

Steven Rostedt <rostedt at goodmis.org> writes:

> On Sun, 28 Apr 2019 10:41:10 -0700
> Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>> > Note that at any given point
>> > in time, there can be at most four such call insn emulations pending:
>> > namely at most one per "process", "irq", "softirq" and "nmi" context.
>> >   
>> 
>> That’s quite an assumption. I think your list should also contain
>> exception, exceptions nested inside that exception, and machine
>> check, at the very least.  I’m also wondering why irq and softirq are
>> treated separately.

You're right, I missed the machine check case.


> 4 has usually been the context count we choose. But I guess in theory,
> if we get exceptions then I could potentially be more.

After having seen the static_call discussion, I'm in no way defending
this limited approach here, but out of curiosity: can the code between
the push onto the stack from ftrace_int3_handler() and the subsequent
pop from the stub actually trigger an (non-#MC) exception? There's an
iret inbetween, but that can fault only when returning to user space,
correct?


> As for irq vs softirq, an interrupt can preempt a softirq. Interrupts
> are enabled while softirqs are running. When sofirqs run, softirqs are
> disabled to prevent nested softirqs. But interrupts are enabled again,
> and another interrupt may come in while a softirq is executing.
>

Thanks,

Nicolai


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