[PATCH] Fix RWMutex to be thread-safe when pthread_rwlock is not available

[Mark Seaborn]

On 26 February 2014 17:30, Chandler Carruth <chandlerc at gmail.com> wrote:

>
>   It's not trivial, but I'm moderately familiar with how to implement a
> reasonable RW mutex on top of a mutex. Would you prefer that? I'm happy to
> contribute such an implementation.


Thanks -- I'm familiar with how to do that too, and I might add a
pthread_rwlock implementation to nacl-newlib's libpthread at some point.
 It seems like overkill to add a rwlock implementation to LLVM, though,
because it would currently only get used under NaCl, and only for a very
small number of uses in LLVM.

I just didn't want to leave lib/Support/Unix/RWMutex.cpp as a
silently-unsafe fallback code path, because that could cause nasty
surprises for other people in the future.  Another option here would just
be to have writer_acquire() call report_fatal_error(), but it seemed better
to add a simple but non-optimal implementation that works. :-)

Cheers,
Mark
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