RE: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

[David Laight]

....
> > 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

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