I believe that the features in C11 will prove quite helpful, though opinions do vary rather widely (disclaimer: I was involved in the C11 process). But please keep in mind that there has been concurrent code written in C for some decades: Sequent started in 1983, which was almost three decades ago, and Sequent was probably not the first to write parallel code in C. Linux added SMP support in the mid-1990s, which is well over a decade ago.
So what did us parallel programmers do during the decades before C11? Initially, we relied on the fact that early C compilers did not do aggressive optimizations, though register reloads did sometimes trip us up. More recently, we have relied on non-standard extensions (e.g., the barrier() macro) and volatile casts. Those of us living through that time (and I am a relative newcomer, starting shared-memory parallel programming only in 1990) have been shifting our coding practices as the compiler optimizations become more aggressive.
And I am very sorry to report that I really have seen compiler writers joyfully discuss how new optimizations will break existing code. In their defense, yes, the code that they were anticipating breaking was relying on undefined behavior. Then again, until production-quality C11 compilers become widely available, all multithreaded C programs that allow concurrent read-write access to any given variable will continue to rely on undefined behavior.
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