linux-kernel - Re: kprobe: __blkdev

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Message-Id: <20200623084706.e6e99e99d7da6690e7a6c199@kernel.org>
Date:   Tue, 23 Jun 2020 08:47:06 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ming Lei <ming.lei@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ming Lei <tom.leiming@...il.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-block <linux-block@...r.kernel.org>
Subject: Re: kprobe: __blkdev_put probe is missed

On Mon, 22 Jun 2020 09:01:48 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 22 Jun 2020 08:27:53 +0800
> Ming Lei <ming.lei@...hat.com> wrote:
> 
> > Can you kprobe guys improve the implementation for covering this case?
> > For example, put probe on 3) in case the above situation is recognized.
> 
> To do so would require solving the halting problem.
> 
>   https://en.wikipedia.org/wiki/Halting_problem
> 
> Or perhaps reading the DWARF output of the compiler to determine if it
> optimized the location you are looking for.

As far as I can see, gcc-9.3 doesn't generate this information :(
Maybe the optimizer forgot to push the tail-call callsite information
to dwarf generator when making a recursive tail-call to a loop.

> The first case is impossible to solve, the second would take a lot of
> work, (are you going to fund it?)

What I can provide is "--skip-prologue" option for the perf-probe
which will be similar to the "-P" option. If the compiler correctly
generates the information, we can enable it automatically. But
as far as I can see, it doesn't.

[OT] DWARF has its option(and GNU extension) but it seems not correctly
implemented yet.

http://www.dwarfstd.org/ShowIssue.php?issue=100909.2

What I found on __blkdev_put and the callers, the "tail-call to other
function" were recorded as a call-site DIE with DW_AT_tail_call. But
if the "tail-call to itself (recursive call)" was expanded as a loop,
it was just disappeared. No call-site information, nor the DW_AT_tail_call.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>