linux-kernel - Re: [PATCH 6/8] trace_uprobe/sdt: Fix multiple update of same reference counter

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Message-ID: <20180319134050.GA12554@redhat.com>
Date:   Mon, 19 Mar 2018 14:40:51 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Cc:     mhiramat@...nel.org, peterz@...radead.org,
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Subject: Re: [PATCH 6/8] trace_uprobe/sdt: Fix multiple update of same
 reference counter

Hi Ravi,

On 03/19, Ravi Bangoria wrote:
>
> On 03/16/2018 11:20 PM, Oleg Nesterov wrote:
> >
> > And it seems that you are trying to confuse yourself, not only me ;) Just
> > suppose that an application does mmap+munmap in a loop and the mapped region
> > contains uprobe but not the counter.
>
> this is fine because ...

Yes, I guess I tried to say "counter but not uprobe" but possibly I was actually
confused.

> Our initial design was to increment counter in install_breakpoint() but
> uprobed instruction gets patched in a very early stage of binary loading
> and vma that holds the counter may not be mapped yet.

Yes, yes, I understand this is not that simple...

> > Btw, why do we need a counter, not a boolean? Who else can modify it?
> > Or different uprobes can share the same counter?
>
> Yes, multiple SDT markers can share the counter.

OK, thanks.

Oleg.