[llvm-bugs] [Bug 35464] New: Crash at llvm_gcda_emit_arcs or llvm_gcda_emit_function when running multiple processes

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35464

            Bug ID: 35464
           Summary: Crash at llvm_gcda_emit_arcs or
                    llvm_gcda_emit_function when running multiple
                    processes
           Product: compiler-rt
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: compiler-rt
          Assignee: unassignedbugs at nondot.org
          Reporter: mcastelluccio at mozilla.com
                CC: llvm-bugs at lists.llvm.org

There's still something that causes inconsistencies when multiple processes are
writing the same gcda files.

I have noticed these crashes, intermittently, in Firefox xpcshell and jittest
suites, which are running several instances of Firefox in parallel.

The crash usually occurs either in llvm_gcda_emit_arcs or in
llvm_gcda_emit_function, when a process opens a file that has already been
written by another. It never happens when the file is new.

I have a patch that seems to fix this problem. The fix is to flush the file
buffers before unlocking the file. Currently, the flush is done with fclose
after the file is unlocked. So, between the unlocking and the flushing, there
might be other processes opening the file and mapping it in memory when it
hasn't been completely written yet. So, the process ends up writing outside the
mapped memory and crashes.

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