Thursday, January 24, 2019

Volatile considered harmful

I happened on this today.  The article is narrowly-focused on Linux kernel work, but in my mind it helps to clarify a lot of "volatile" debate I've seen over the years.

I will note that when Corbet (the author) says, "the 'volatile' type class should not be used", what he really means is that you should not declare variables with volatile (or rather, almost never).  Corbet says, "the kernel primitives which make concurrent access to data safe ... If they are used properly, there will be no need to use volatile as well."  Some of those kernel primitives use volatile, but not in variable declarations.  Instead they use volatile in carefully-selected casts.

For example, as described in another Corbet article, he talks about another kernel primitive, "ACCESS_ONCE()".  It is defined as:

    #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

The variable being accessed is temporarily cast to volatile to allow code to be written that violates threading assumptions made by the compiler's optimizer.  Like here, for example.  :-)

I only bring this up to point out that Corbet is not arguing that volatile should never be used at all.  Rather he is arguing that programmers (specifically kernel programmers) should not declare variables to be volatile.  Generally, programmers should use threading primitives to ensure correct code, and if the code's requirements prevent the use of the usual threading primitives, then lower-level primitives (like "ACCESS_ONCE()") should be used to precisely target volatile's use.