linux-kernel - Re: [PATCH v3 6/9] trace_uprobe: Support SDT markers having reference count (semaphore)

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Message-Id: <20180508005651.45553d3cf72521481d16b801@kernel.org>
Date:   Tue, 8 May 2018 00:56:51 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     oleg@...hat.com, peterz@...radead.org, srikar@...ux.vnet.ibm.com,
        rostedt@...dmis.org, acme@...nel.org, ananth@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, alexander.shishkin@...ux.intel.com,
        alexis.berlemont@...il.com, corbet@....net,
        dan.j.williams@...el.com, jolsa@...hat.com, kan.liang@...el.com,
        kjlx@...pleofstupid.com, kstewart@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, milian.wolff@...b.com, mingo@...hat.com,
        namhyung@...nel.org, naveen.n.rao@...ux.vnet.ibm.com,
        pc@...ibm.com, tglx@...utronix.de, yao.jin@...ux.intel.com,
        fengguang.wu@...el.com, jglisse@...hat.com
Subject: Re: [PATCH v3 6/9] trace_uprobe: Support SDT markers having
 reference count (semaphore)

On Mon, 7 May 2018 13:51:21 +0530
Ravi Bangoria <ravi.bangoria@...ux.ibm.com> wrote:

> Hi Masami,
> 
> On 05/04/2018 07:51 PM, Ravi Bangoria wrote:
> >
> >>> +}
> >>> +
> >>> +static void sdt_increment_ref_ctr(struct trace_uprobe *tu)
> >>> +{
> >>> +	struct uprobe_map_info *info;
> >>> +
> >>> +	uprobe_down_write_dup_mmap();
> >>> +	info = uprobe_build_map_info(tu->inode->i_mapping,
> >>> +				tu->ref_ctr_offset, false);
> >>> +	if (IS_ERR(info))
> >>> +		goto out;
> >>> +
> >>> +	while (info) {
> >>> +		down_write(&info->mm->mmap_sem);
> >>> +
> >>> +		if (sdt_find_vma(tu, info->mm, info->vaddr))
> >>> +			sdt_update_ref_ctr(info->mm, info->vaddr, 1);
> >> Don't you have to handle the error to map pages here?
> > Correct.. I think, I've to feedback error code to probe_event_{enable|disable}
> > and handler failure there.
> 
> I looked at this. Actually, It looks difficult to feedback errors to
> probe_event_{enable|disable}, esp. in the mmap() case.

Hmm, can't you roll that back if sdt_increment_ref_ctr() fails?
If so, how does sdt_decrement_ref_ctr() work in that case?

> Is it fine if we just warn sdt_update_ref_ctr() failures in dmesg? I'm
> doing this in [PATCH 7]. (Though, it makes more sense to do that in
> [PATCH 6], will change it in next version).

Of course we need to warn it at least, but the best is rejecting to
enable it.

> 
> Any better ideas?
> 
> BTW, same issue exists for normal uprobe. If uprobe_mmap() fails,
> there is no feedback to trace_uprobe and no warnigns in dmesg as
> well !! There was a patch by Naveen to warn such failures in dmesg
> but that didn't go in: https://lkml.org/lkml/2017/9/22/155

Oops, that's a real bug. It seems the ball is in Naveen's hand.
Naveen, could you update it according to Oleg's comment, and resend it?

> 
> Also, I'll add a check in sdt_update_ref_ctr() to make sure reference
> counter never goes to negative incase increment fails but decrement
> succeeds. OTOH, if increment succeeds but decrement fails, the
> counter remains >0 but there is no harm as such, except we will
> execute some unnecessary code.

I see. Please carefully clarify whether such case is kernel's bug or not.
I would like to know what the condition causes that uneven behavior.

Thank you,

> 
> Thanks,
> Ravi
> 


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