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

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Date:   Fri, 16 Mar 2018 18:50:30 +0100
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, acme@...nel.org,
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Subject: Re: [PATCH 6/8] trace_uprobe/sdt: Fix multiple update of same
 reference counter

On 03/16, Ravi Bangoria wrote:
>
> On 03/15/2018 08:19 PM, Oleg Nesterov wrote:
> > On 03/13, Ravi Bangoria wrote:
> >> For tiny binaries/libraries, different mmap regions points to the
> >> same file portion. In such cases, we may increment reference counter
> >> multiple times.
> > Yes,
> >
> >> But while de-registration, reference counter will get
> >> decremented only by once
> > could you explain why this happens? sdt_increment_ref_ctr() and
> > sdt_decrement_ref_ctr() look symmetrical, _decrement_ should see
> > the same mappings?

...

>     # strace -o out python
>       mmap(NULL, 2738968, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fff92460000
>       mmap(0x7fff926a0000, 327680, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x230000) = 0x7fff926a0000
>       mprotect(0x7fff926a0000, 65536, PROT_READ) = 0

Ah, in this case everything is clear, thanks.

I was confused by the changelog, I misinterpreted it as if inc/dec are not
balanced in case of multiple mappings even if the application doesn't play
with mmap/mprotect/etc.

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.

And this all makes me think that we should do something else. Ideally,
install_breakpoint() and remove_breakpoint() should inc/dec the counter
if they do not fail...

Btw, why do we need a counter, not a boolean? Who else can modify it?
Or different uprobes can share the same counter?

Oleg.