[PATCH] D29588: Use rw locks for sanitizer thread registry

[Dmitry Vyukov via Phabricator via llvm-commits]

dvyukov added a comment.

In https://reviews.llvm.org/D29588#669125, @fjricci wrote:

> In https://reviews.llvm.org/D29588#668828, @dvyukov wrote:
>
> > > Sharing a mutex access across threads like this is not supported by blocking mutex
> >
> > Why?
>
>
> If a thread owns a mutex on the thread registry for writing (as with a blocking mutex), we can't really assume in another thread that the registry won't change. Since the thread accessing the data is actually in a separate process (the result of a clone() in StopTheWorld), I'm not sure that there's even a way to pass the blocking mutex around safely.
>
> > I've checked BlockingMutex comments and linux/mac/win implementation. I don't see any indication that it's not supported.
> >  How is RWMutex different?
>
> An RWMutex acquired for read-only use can be safely used for reads by multiple threads, as it guarantees the protected data won't change. A blocking mutex provides no guarantees that the protected state won't change (and in fact implies that it probably will).


Sorry, I am not following you.
How can protected data change when a thread holds the mutex and is not changing the data? Who/how can change the data?
Whoever wants to change the data must acquire the mutex first, but it won't because the parent process holds the mutex.
What do you mean y "passing blocking mutex around"? And why do we need to do it?

The parent process holds the mutex thus ensuring that  nobody else is changing the data.


https://reviews.llvm.org/D29588