Add a warning that Miri does not check for data-races.

Author: Vytautas Astrauskas

README.md

@@ -42,7 +42,9 @@ program, and cannot run all programs:
 * Miri runs the program as a platform-independent interpreter, so the program
   has no access to most platform-specific APIs or FFI. A few APIs have been
   implemented (such as printing to stdout) but most have not: for example, Miri
-  currently does not support concurrency, or SIMD, or networking.
+  currently does not support SIMD or networking.
+* Miri currently does not check for data-races and other concurrency related
+  issues.
 
 [rust]: https://www.rust-lang.org/
 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md

src/shims/foreign_items/posix.rs

@@ -255,6 +255,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
             // Threading
             "pthread_create" => {
+                println!("WARNING: The thread support is experimental. \
+                          For example, Miri does not detect data races yet.");
                 assert_eq!(args.len(), 4);
                 let func = args[2];
                 let fn_ptr = this.read_scalar(func)?.not_undef()?;