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

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Date:   Thu, 24 May 2018 18:26:08 +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, Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v3 6/9] trace_uprobe: Support SDT markers having
 reference count (semaphore)

Hi Ravi,

sorry for delay!

I am trying to recall what this code should do ;) At first glance, I do
not see any serious problem in this version... except it doesn't apply
to Linus's tree. just one question for now.

On 04/17, Ravi Bangoria wrote:
>
> @@ -941,6 +1091,9 @@ typedef bool (*filter_func_t)(struct uprobe_consumer *self,
>  	if (ret)
>  		goto err_buffer;
>  
> +	if (tu->ref_ctr_offset)
> +		sdt_increment_ref_ctr(tu);
> +

iiuc, this is probe_event_enable()...

Looks racy, but afaics the race with uprobe_mmap() will be closed by the next
change. However, it seems that probe_event_disable() can race with trace_uprobe_mmap()
too and the next 7/9 patch won't help,

> +	if (tu->ref_ctr_offset)
> +		sdt_decrement_ref_ctr(tu);
> +
>  	uprobe_unregister(tu->inode, tu->offset, &tu->consumer);
>  	tu->tp.flags &= file ? ~TP_FLAG_TRACE : ~TP_FLAG_PROFILE;

so what if trace_uprobe_mmap() comes right after uprobe_unregister() ?
Note that trace_probe_is_enabled() is T until we update tp.flags.

Oleg.