[PATCH xserver 4/8] Create a threaded mechanism for input [v3]

[Keith Packard]

Mark Kettenis <mark.kettenis at xs4all.nl> writes:

> It isn't odd, because PTHREAD_RECURSIVE_MUTEX_INITIALIZE isn't in the
> standard.  You'll have to explicitly initialize the mutex with
> pthread_mutex_init() to get a recursive mutex.

Sigh. That's a pain in this case; the first use of the mutex occurs
before the DDX initializes the input thread. I'll use
PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP where it exists, and fallback to
a test inside input_lock() that initializes the mutex on first
use. That's "safe" because the first use will always happen before the
input thread is started.

> You can always prove a point with an appropriately constructed
> microbenchmark ;).  Seriously though, I do hope that the overhead of
> the recursive mutex isn't noticable in the Xorg input thread.

I was surprised at how bad the Linux recursive mutexes performed
compared with a thread-local counter, if you actually used recursion. In
the (we hope common) case where the mutex gets acquired only once, they
are the same.

Depth   Recursive       Counter
10      30.3ns          13.2ns 
1       100.4ns         100.4ns

> Anyway, the diff below won't fly as
> PTHREAD_RECURSIVE_MUTEX_INITIALIZER isn't in the standard and
> therefore not widely available.  On OpenBSD we don't even have
> PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP, and supporting such a thing
> would be difficult.

That's unfortunate, but easy enough to work around with a bit of
conditional code.

Here's code which works only with recursive mutexes, initializing it
explicitly when PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP is not available.

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-- 
-keith
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