linux-kernel - Re: [PATCH v5 06/10] Uprobes: Support SDT markers having reference count (semaphore)

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Message-Id: <bca2cd84-ed4f-7364-687e-b10e3e930b06@linux.ibm.com>
Date:   Wed, 4 Jul 2018 14:54:25 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, rostedt@...dmis.org,
        mhiramat@...nel.org, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-doc@...r.kernel.org, ananth@...ux.vnet.ibm.com,
        alexis.berlemont@...il.com, naveen.n.rao@...ux.vnet.ibm.com,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v5 06/10] Uprobes: Support SDT markers having reference
 count (semaphore)

Hi Srikar,

On 07/04/2018 02:46 PM, Srikar Dronamraju wrote:
>>
>> I'm not sure if I get your concerns but let me try to explain what happens
>> in such cases. please let me know if I misunderstood your point.
>>
>> 1. Install a probe using perf.
>>   # ./perf probe sdt_tick:loop2
>>
>>
>>
>> Does this explain your concerns?
>>
> 
> 
> No, this was not my concern.
> My concern is with two users on the same USDT.
> 1. First user enables the probe point but doesn't increment the ref_cnt.
> via uprobe_register
> 
> 2. Second user tries to enable the probe point and also increments the
> ref_cnt via uprobe_register_refctr.


Ok got it. uprobe_register_refctr() will return with error because we don't
allow one uprobe(inode+offset) with multiple reference counter.

i.e. If inode+offset matches for two uprobes, ref_ctr_offset must match as
well. Patch 8/10 takes care of this.


> 
> 3. If the second user now removes the probe point via uprobe_unregister.
> 
> 4. What is the state of the ref_cnt?
> 
> --
> Thanks and Regards
> Srikar
>