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Re: mmap'ed robust mutexes and possible undefined behaviour
- From: Florian Weimer <fw at deneb dot enyo dot de>
- To: "Carlos O'Donell" <carlos at redhat dot com>
- Cc: Marcos Dione <mdione at grulic dot org dot ar>, libc-help at sourceware dot org
- Date: Tue, 25 Nov 2014 20:25:39 +0100
- Subject: Re: mmap'ed robust mutexes and possible undefined behaviour
- Authentication-results: sourceware.org; auth=none
- References: <20141124203441 dot GA3759 at diablo dot grulicueva dot local> <5473A7B0 dot 5050803 at redhat dot com>
* Carlos O'Donell:
>> 1. we open a file.
>> 2. we mmap it and use that mem to store a robust mutex.
>> 3. we lock the mutex.
>> 4. we munmap the file.
>> 5. we close the file.
> Undefined behaviour.
> This results in undefined behaviour since the allocated storage for
> the mutex object has been lost. You need to keep that storage around
> for the robust algorithms to work with. Without any data you can't
> do anything.
Some might expect that the mutex enters inconsistent state when the
mutex is unmapped, not just on process death. Then the data would
still be available at the time the state change would need to happen.