linux-kernel - Re: [PATCH 2/3] tools: testing: Use existing atomic.h for vma/radix-tree tests

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Message-ID: <DCDB7IYZS73V.2KL54A53D1O1L@google.com>
Date: Wed, 27 Aug 2025 15:19:12 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Jann Horn <jannh@...gle.com>, <linux-kernel@...r.kernel.org>, 
	<maple-tree@...ts.infradead.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/3] tools: testing: Use existing atomic.h for
 vma/radix-tree tests

On Wed Aug 27, 2025 at 12:56 PM UTC, Pedro Falcato wrote:
> On Wed, Aug 27, 2025 at 11:04:42AM +0000, Brendan Jackman wrote:
>> The shared userspace logic used for unit-testing radix-tree and VMA code
>> currently has its own replacements for atomics helpers. This is not
>> needed as the necessary APIs already have userspace implementations in
>> the tools tree. Switching over to that allows deleting a bit of code.
>> 
>> Note that the implementation is different; while the version being
>> deleted here is implemented using liburcu, the existing version in tools
>> uses either x86 asm or compiler builtins. It's assumed that both are
>> equally likely to be correct.
>> 
>> The tools tree's version of atomic_t is a struct type while the version
>> being deleted was just a typedef of an integer. This means it's no
>> longer valid to call __sync_bool_compare_and_swap() directly on it. One
>> option would be to just peek into the struct and call it on the field,
>> but it seems a little cleaner to just use the corresponding atomic.h
>> API. On non-x86 archs this is implemented using
>> __sync_val_compare_and_swap(). It's not clear why the old version uses
>> the bool variant instead of the generic "val" one, for now it's assumed
>> that this was a mistake.
>>
>
> I don't think it's a mistake. Namely we're checking if the cmpxchg occured.
> So in the new version you'll have trouble incrementing i_mmap_writeable from
> 0 to 1, where in practice you should (AIUI) see 0 -> 1 (old val = 0, retry) -> 2,
> which is obviously not correct here.
>
> At the very least you'll need some:
>
> do {
> } while(atomic_cmpxchg(&mapping->i_mmap_writeable, c, c+1) != c);

Oops, yeah my code is total nonsense here - thanks for paying attention.
I guess I "got away with it" because there's probably no actual races
going on when I run the tests...

Anyway I'll apply my brain properly next time I get the chance and send
a v2.

Thanks for the review!