linux-kernel - Re: kprobe: __blkdev

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Date:   Tue, 23 Jun 2020 09:38:01 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ming Lei <ming.lei@...hat.com>,
        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 Tue, 23 Jun 2020 08:47:06 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> 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

Oops, sorry, I missed the following sentences.

"Tail calls are jump-like instructions which transfer control to the start
of some subprogram, but the call site location address isn't visible in the
unwind information."

"Tail recursion is a call to the current function which is compiled as a
loop into the middle of the current function."

"The DW_TAG_call_site entries describe normal and tail calls."

This means, the gcc is correctly implemented and this __blkdev_put() case
is NOT covered by DT_TAG_call_site.
So we can not detect it from the debuginfo.

Thank you,

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