linux-kernel - Re: [PATCH v2 7/9] trace_uprobe/sdt: Fix multiple update of same reference counter

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Message-Id: <c3bd7507-977e-67b5-9ab0-d70b4908a6f2@linux.vnet.ibm.com>
Date:   Wed, 11 Apr 2018 09:58:13 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     mhiramat@...nel.org, 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,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 7/9] trace_uprobe/sdt: Fix multiple update of same
 reference counter

Hi Oleg,

On 04/10/2018 04:36 PM, Oleg Nesterov wrote:
> Hi Ravi,
>
> On 04/10, Ravi Bangoria wrote:
>>> and what if __mmu_notifier_register() fails simply because signal_pending() == T?
>>> see mm_take_all_locks().
>>>
>>> at first glance this all look suspicious and sub-optimal,
>> Yes. I should have added checks for failure cases.
>> Will fix them in v3.
> And what can you do if it fails? Nothing except report the problem. But
> signal_pending() is not the unlikely or error condition, it should not
> cause the tracing errors.

...

> Plus mm_take_all_locks() is very heavy... BTW, uprobe_mmap_callback() is
> called unconditionally. Whatever it does, can we at least move it after
> the no_uprobe_events() check? Can't we also check MMF_HAS_UPROBES?

Sure, I'll move it after these conditions.

> Either way, I do not feel that mmu_notifier is the right tool... Did you
> consider the uprobe_clear_state() hook we already have?

Ah! This is really a good idea. We don't need mmu_notifier then.

Thanks for suggestion,
Ravi