Re: [RESEND PATCH v2] devres: Really align data field to unsigned long long

[Alexey Brodkin]

Hi Vineet,

On Mon, 2018-07-09 at 11:27 -0700, Vineet Gupta wrote:
> On 07/09/2018 03:23 AM, Alexey Brodkin wrote:
> > Hi David,
> > 
> > On Mon, 2018-07-09 at 10:18 +0000, David Laight wrote:
> > > From: Alexey Brodkin
> > > > Sent: 09 July 2018 11:00
> > > 
> > > ...
> > > > That's a good idea indeed but it doesn't solve the problem with
> > > > struct devres_node. Consider the following snippet:
> > > > -------------------------------->8-------------------------------
> > > > 	struct mystruct {
> > > > 		atomic64_t myvar;
> > > > 	}
> > > > 
> > > > 	struct mystruct *p;
> > > > 	p = devm_kzalloc(dev, sizeof(*p), GFP_KERNEL);
> > > > -------------------------------->8-------------------------------
> > > > 
> > > > Here myvar address will match address of "data" member of struct devres_node.
> > > > So if "data" is has offset of 12 bytes from the beginning of a page then
> > > > myvar won't be 64-bit aligned regardless of myvar's attribute, right?
> > > 
> > > ...
> > > > > > -	unsigned long long		data[];	/* guarantee ull alignment */
> > > 
> > > Ahh, that line should be:
> > > 	u8 data[] __aligned(8); /* Guarantee 64bit alignment */
> > 
> > And that pretty much what I suggested in my initial patch :)
> > 
> > For the record x86 has exactly the same atomic64_t as you suggested,
> > see https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/atomic64_32.h#L13:
> > ---------------------->8------------------
> > typedef struct {
> > 	u64 __aligned(8) counter;
> > } atomic64_t;
> > ---------------------->8------------------
> 
> And so does the ARC version since when the atomic64_t support was added by commit
> ce6365270ecd
> 
> Also, you should consider using the pre-canned type aligned_u64.
> 
> typedef struct {
>        aligned_u64 counter;
>        ^^^^^^^^^^^
> } atomic64_t;

As Peter mentioned earlier atomic is signed, see:
http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004030.html

So looks like we need this fix for ARC instead:
--------------------------------->8----------------------------
diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
index 11859287c52a..ef94b7371de6 100644
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -367,7 +367,7 @@ ATOMIC_OPS(xor, ^=, CTOP_INST_AXOR_DI_R2_R2_R3)
  */
 
 typedef struct {
-       aligned_u64 counter;
+       long long __aligned(8) counter;
 } atomic64_t;
 
 #define ATOMIC64_INIT(a) { (a) }
--------------------------------->8----------------------------

-Alexey