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

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Date:   Tue, 10 Apr 2018 13:06:33 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.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
Subject: Re: [PATCH v2 7/9] trace_uprobe/sdt: Fix multiple update of same
 reference counter

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?

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

Oleg.