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Date:   Thu, 24 May 2018 18:26:08 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc:     mhiramat@kernel.org, peterz@infradead.org,
        srikar@linux.vnet.ibm.com, rostedt@goodmis.org, acme@kernel.org,
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        yao.jin@linux.intel.com, fengguang.wu@intel.com,
        jglisse@redhat.com, Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Subject: Re: [PATCH v3 6/9] trace_uprobe: Support SDT markers having
 reference count (semaphore)
Message-ID: <20180524162608.GA27082@redhat.com>
References: <20180417043244.7501-1-ravi.bangoria@linux.vnet.ibm.com>
 <20180417043244.7501-7-ravi.bangoria@linux.vnet.ibm.com>
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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.