Dean Michael Berris 57f2739291 [XRay] Use optimistic logging model for FDR mode ... Summary: Before this change, the FDR mode implementation relied on at thread-exit handling to return buffers back to the (global) buffer queue. This introduces issues with the initialisation of the thread_local objects which, even through the use of pthread_setspecific(...) may eventually call into an allocation function. Similar to previous changes in this line, we're finding that there is a huge potential for deadlocks when initialising these thread-locals when the memory allocation implementation is also xray-instrumented. In this change, we limit the call to pthread_setspecific(...) to provide a non-null value to associate to the key created with pthread_key_create(...). While this doesn't completely eliminate the potential for the deadlock(s), it does allow us to still clean up at thread exit when we need to. The change is that we don't need to do more work when starting and ending a thread's lifetime. We also have a test to make sure that we actually can safely recycle the buffers in case we end up re-using the buffer(s) available from the queue on multiple thread entry/exits. This change cuts across both LLVM and compiler-rt to allow us to update both the XRay runtime implementation as well as the library support for loading these new versions of the FDR mode logging. Version 2 of the FDR logging implementation makes the following changes: * Introduction of a new 'BufferExtents' metadata record that's outside of the buffer's contents but are written before the actual buffer. This data is associated to the Buffer handed out by the BufferQueue rather than a record that occupies bytes in the actual buffer. * Removal of the "end of buffer" records. This is in-line with the changes we described above, to allow for optimistic logging without explicit record writing at thread exit. The optimistic logging model operates under the following assumptions: * Threads writing to the buffers will potentially race with the thread attempting to flush the log. To avoid this situation from occuring, we make sure that when we've finalized the logging implementation, that threads will see this finalization state on the next write, and either choose to not write records the thread would have written or write the record(s) in two phases -- first write the record(s), then update the extents metadata. * We change the buffer queue implementation so that once it's handed out a buffer to a thread, that we assume that buffer is marked "used" to be able to capture partial writes. None of this will be safe to handle if threads are racing to write the extents records and the reader thread is attempting to flush the log. The optimism comes from the finalization routine being required to complete before we attempt to flush the log. This is a fairly significant semantics change for the FDR implementation. This is why we've decided to update the version number for FDR mode logs. The tools, however, still need to be able to support older versions of the log until we finally deprecate those earlier versions. Reviewers: dblaikie, pelikan, kpw Subscribers: llvm-commits, hiraditya Differential Revision: https://reviews.llvm.org/D39526 llvm-svn: 318733		2017-11-21 07:16:57 +00:00
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CMakeLists.txt
InstrumentationMap.cpp
Trace.cpp	[XRay] Use optimistic logging model for FDR mode	2017-11-21 07:16:57 +00:00