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Subject: Re: [PATCH v2 7/9] trace_uprobe/sdt: Fix multiple update of same
 reference counter
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     mhiramat@kernel.org, peterz@infradead.org,
        srikar@linux.vnet.ibm.com, rostedt@goodmis.org, acme@kernel.org,
        ananth@linux.vnet.ibm.com, akpm@linux-foundation.org,
        alexander.shishkin@linux.intel.com, alexis.berlemont@gmail.com,
        corbet@lwn.net, dan.j.williams@intel.com, jolsa@redhat.com,
        kan.liang@intel.com, kjlx@templeofstupid.com,
        kstewart@linuxfoundation.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        milian.wolff@kdab.com, mingo@redhat.com, namhyung@kernel.org,
        naveen.n.rao@linux.vnet.ibm.com, pc@us.ibm.com, tglx@linutronix.de,
        yao.jin@linux.intel.com, fengguang.wu@intel.com,
        jglisse@redhat.com,
        Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
References: <20180404083110.18647-1-ravi.bangoria@linux.vnet.ibm.com>
 <20180404083110.18647-8-ravi.bangoria@linux.vnet.ibm.com>
 <20180409132928.GA25722@redhat.com>
 <84c1e60f-8aad-a0ce-59af-4fcb3f77df94@linux.vnet.ibm.com>
 <20180410110633.GA29063@redhat.com>
From:   Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Date:   Wed, 11 Apr 2018 09:58:13 +0530
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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