[all-commits] [llvm/llvm-project] d137db: [TSan][libdispatch] Add interceptors for dispatch_...

[Julian Lettner via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: d137db80297f286f3a19eacc63d4a980646da437
      https://github.com/llvm/llvm-project/commit/d137db80297f286f3a19eacc63d4a980646da437
  Author: Julian Lettner <julian.lettner at apple.com>
  Date:   2020-08-14 (Fri, 14 Aug 2020)

  Changed paths:
    M compiler-rt/lib/tsan/rtl/tsan_interceptors_libdispatch.cpp
    A compiler-rt/test/tsan/libdispatch/async_and_wait.c

  Log Message:
  -----------
  [TSan][libdispatch] Add interceptors for dispatch_async_and_wait()

Add interceptors for `dispatch_async_and_wait[_f]()` which was added in
macOS 10.14.  This pair of functions is similar to `dispatch_sync()`,
but does not force a context switch of the queue onto the caller thread
when the queue is active (and hence is more efficient).  For TSan, we
can apply the same semantics as for `dispatch_sync()`.

>From the header docs:
> Differences with dispatch_sync()
>
> When the runtime has brought up a thread to invoke the asynchronous
> workitems already submitted to the specified queue, that servicing
> thread will also be used to execute synchronous work submitted to the
> queue with dispatch_async_and_wait().
>
> However, if the runtime has not brought up a thread to service the
> specified queue (because it has no workitems enqueued, or only
> synchronous workitems), then dispatch_async_and_wait() will invoke the
> workitem on the calling thread, similar to the behaviour of functions
> in the dispatch_sync family.

Additional context:
> The guidance is to use `dispatch_async_and_wait()` instead of
> `dispatch_sync()` when it is necessary to mix async and sync calls on
> the same queue. `dispatch_async_and_wait()` does not guarantee
> execution on the caller thread which allows to reduce context switches
> when the target queue is active.
> https://gist.github.com/tclementdev/6af616354912b0347cdf6db159c37057

rdar://35757961

Reviewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D85854