Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with load-acquire and store-release

[Adrian Hunter <adrian.hunter@intel.com>]

On 1/06/21 9:33 am, Leo Yan wrote:
> On Mon, May 31, 2021 at 10:03:33PM +0300, Adrian Hunter wrote:
>> On 31/05/21 6:10 pm, Leo Yan wrote:
>>> Hi Peter, Adrian,
>>>
>>> On Wed, May 19, 2021 at 10:03:19PM +0800, Leo Yan wrote:
>>>> Load-acquire and store-release are one-way permeable barriers, which can
>>>> be used to guarantee the memory ordering between accessing the buffer
>>>> data and the buffer's head / tail.
>>>>
>>>> This patch optimizes the memory ordering with the load-acquire and
>>>> store-release barriers.
>>>
>>> Is this patch okay for you?
>>>
>>> Besides this patch, I have an extra question.  You could see for
>>> accessing the AUX buffer's head and tail, it also support to use
>>> compiler build-in functions for atomicity accessing:
>>>
>>>   __sync_val_compare_and_swap()
>>>   __sync_bool_compare_and_swap()
>>>
>>> Since now we have READ_ONCE()/WRITE_ONCE(), do you think we still need
>>> to support __sync_xxx_compare_and_swap() atomicity?
>>
>> I don't remember, but it seems to me atomicity is needed only
>> for a 32-bit perf running with a 64-bit kernel.
> 
> 32-bit perf wants to access 64-bit value atomically, I think it tries to
> avoid the issue caused by scenario:
> 
>         CPU0 (64-bit kernel)           CPU1 (32-bit user)
> 
>                                          read head_lo
>         WRITE_ONCE(head)
>                                          read head_hi
> 
> 
> I dumped the disassembly for reading 64-bit value for perf Arm32 and get
> below results:
> 
>   perf Arm32 for READ_ONCE():
> 
> 	case 8: *(__u64_alias_t *) res = *(volatile __u64_alias_t *) p; break;
>      84a:	68fb      	ldr	r3, [r7, #12]
>      84c:	e9d3 2300 	ldrd	r2, r3, [r3]
>      850:	6939      	ldr	r1, [r7, #16]
>      852:	e9c1 2300 	strd	r2, r3, [r1]
>      856:	e007      	b.n	868 <auxtrace_mmap__read_head+0xb0>
> 
> It uses the instruction ldrd which is "Load Register Dual (register)",
> but this doesn't mean the instruction is atomic, especially based on
> the comment in the kernel header include/asm-generic/rwonce.h, I think
> the instruction ldrd/strd will be "atomic in some cases (namely Armv7 +
> LPAE), but for others we rely on the access being split into 2x32-bit
> accesses".
> 
> 
>   perf Arm32 for __sync_val_compare_and_swap():
> 
> 	u64 head = __sync_val_compare_and_swap(&pc->aux_head, 0, 0);
>      7d6:	68fb      	ldr	r3, [r7, #12]
>      7d8:	f503 6484 	add.w	r4, r3, #1056	; 0x420
>      7dc:	f04f 0000 	mov.w	r0, #0
>      7e0:	f04f 0100 	mov.w	r1, #0
>      7e4:	f3bf 8f5b 	dmb	ish
>      7e8:	e8d4 237f 	ldrexd	r2, r3, [r4]
>      7ec:	ea52 0c03 	orrs.w	ip, r2, r3
>      7f0:	d106      	bne.n	800 <auxtrace_mmap__read_head+0x48>
>      7f2:	e8c4 017c 	strexd	ip, r0, r1, [r4]
>      7f6:	f1bc 0f00 	cmp.w	ip, #0
>      7fa:	f1bc 0f00 	cmp.w	ip, #0
>      7fe:	d1f3      	bne.n	7e8 <auxtrace_mmap__read_head+0x30>
>      800:	f3bf 8f5b 	dmb	ish
>      804:	e9c7 2304 	strd	r2, r3, [r7, #16]
> 
> For __sync_val_compare_and_swap(), it uses the instructions
> ldrexd/ldrexd, these two instructions rely on the exclusive monitor
> for accessing 64-bit value, so seems to me this is more reliable way
> for accessing 64-bit value in CPU's 32-bit mode.
> 
> Conclusion: seems to me __sync_xxx_compare_and_swap() should be kept
> in this case rather than using READ_ONCE() for 32-bit building.  Or
> any other suggestions?  Thanks!

__sync_xxx_compare_and_swap is out-of-date now. This page:

https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins

recommends '__atomic' builtins instead.

Since atomics are needed only for the "compat" case (i.e. 32-bit perf with 64-bit kernel)
you could try to find an elegant way to check for a 64-bit kernel, and avoid the atomics
for a 32-bit perf with 32-bit kernel.