The expression has to contain both dereferences, eg, a->b->c, for the problem to appear. If they're separated by using a temp, as in   T * volatile p;   p = a->b;   ... p->c ... then the problem should be avoided. Making the temp "p" volatile is also important, to prevent the compiler from recombining the two dereferences. Or compile with -o1, -o0, or -ooff.

In an expression like A->B->C, when B and C are both volatile structure fields, we'd expect to see two distinct memory accesses every time. There is a bug in the compiler specifically with having two dereferences in a single expression, ie, A->B->C, in which it may lose the volatile qualifier from B. That may lead it to save A->B in a temporary variable and reuse that value instead of re-reading it as is supposed to happen.