.... > > Found a related discussion: > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102714 > > > > Looks like GCC 10, 11 have been backported, not sure whether GCC 8 has been backported. > > > > So, I have the following questions: > > > > Given that some people might not update their GCC, do they need to be notified? > > > > Do we need to CC Linus? > > No need. > > I put the following code into a kernel module: > > typedef struct list_head_shit { > int next; > struct list_head *first; > } list_head_shit; > > static void noinline so_shit(void) { > list_head_shit *head = (list_head_shit *)kmalloc(sizeof(list_head_shit), GFP_KERNEL); > head->first = 0; > head->next = 1; > > READ_ONCE(head->first); > READ_ONCE(head->first); > > kfree(head); > } > > x86_64-linux-gnu-gcc-11 generate the following code: > > 0000000000000000 <so_shit>: > 0: 48 8b 3d 00 00 00 00 mov 0x0(%rip),%rdi # 7 <so_shit+0x7> > 7: ba 10 00 00 00 mov $0x10,%edx > c: be c0 0c 00 00 mov $0xcc0,%esi > 11: e8 00 00 00 00 call 16 <so_shit+0x16> > 16: 48 c7 40 08 00 00 00 movq $0x0,0x8(%rax) > 1d: 00 > 1e: 48 89 c7 mov %rax,%rdi > 21: c7 00 01 00 00 00 movl $0x1,(%rax) > 27: 48 8b 47 08 mov 0x8(%rdi),%rax # READ_ONCE here > 2b: 48 8b 47 08 mov 0x8(%rdi),%rax # READ_ONCE here > 2f: e9 00 00 00 00 jmp 34 <so_shit+0x34> > 34: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1) > 3b: 00 00 00 00 > 3f: 90 nop > > The conclusion is that we can rely on READ_ONCE when writing kernel code. > > The kernel’s READ_ONCE is different with the one Joel wrote yesterday. (Joel’s is the same as the old > ACCESS_ONCE) You do need to reproduce the error with code that looks like the loop in the (old) udp.c code. Then see if changing the implementation of READ_ONCE() from a simple 'volatile' access the newer variant makes a difference. You also need to check with the oldest version of gcc that is still supported - that is much older than gcc 11. In the udp code the volatile access was on a pointer (which should qualify as a scaler type) so it may well be the inlining bug you mentioned earlier, not the 'volatile on non-scaler' feature that READ_ONCE() fixed. That fix hasn't been back-ported to all the versions of gcc that the kernel build supports. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
