On Wed, May 24, 2023, Peter Zijlstra wrote:
On Wed, May 24, 2023 at 01:16:03PM -0700, Sean Christopherson wrote:
Of course, the only accesses outside of mmu_lock are reads, so on x86 that "atomic" access is just a READ_ONCE() load, but that's not the case for all architectures.
This is true on *all* archs. atomic_set() and atomic_read() are no more and no less than WRITE_ONCE() / READ_ONCE().
Ah, I take it s390's handcoded assembly routines are just a paranoid equivalents and not truly special? "l" and "st" do sound quite generic...
commit 7657e41a0bd16c9d8b3cefe8fd5d6ac3c25ae4bf Author: Heiko Carstens hca@linux.ibm.com Date: Thu Feb 17 13:13:58 2011 +0100
[S390] atomic: use inline asm
Use inline assemblies for atomic_read/set(). This way there shouldn't be any questions or subtle volatile semantics left.
static inline int __atomic_read(const atomic_t *v) { int c;
asm volatile( " l %0,%1\n" : "=d" (c) : "R" (v->counter)); return c; }
static inline void __atomic_set(atomic_t *v, int i) { asm volatile( " st %1,%0\n" : "=R" (v->counter) : "d" (i)); }