Re: [PATCH] bitops: fix issue in parallel atomic tests

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-10-13 15:37, Morten Brørup wrote:
>> From: Mattias Rönnblom [mailto:mattias.ronnblom@ericsson.com]
>> Sent: Sunday, 13 October 2024 13.57
>>
>> The macros generating the parallel test for atomic test-and-
>> [set|clear|flip] functions used a 64-bit reference word when assuring
>> no neighbouring bits were modified, even when generating code for the
>> 32-bit version of the test.
>>
>> This issue causes spurious test failures on GCC 12.2.0 (the default
>> compiler on for example Debian 12 "bookworm"), when optimization level
>> 2 or higher are used.
>>
>> The test failures do not occur with GCC 11, 12.3 and 13.2.
>>
>> To the author, this looks like a promotion-related compiler bug in GCC
>> 12.2.
> 
> I am curious about the compiler bug...
> 
> Did the bug occur when the most significant bit was set, so it sign related?
> 

It seems to happen a lot more often than 1/32 times. Also, all involved 
types are unsigned.

If you set the optimization level to "1" (i.e., 
__attribute__((optimize("O"))) on the 
test_bit_atomic_parallel_test_and_modify32 function, the test passes on 
12.2.0.

> Maybe this will reveal something:
> 
> TEST_ASSERT(expected_word == word,
>    "Untouched bits have changed value, %" PRIx ## size
>    " should be %" PRIx64,
>    word, expected_word);
> 

Confusingly enough, the failing assertion is the one prior that assertion.

>>
>> Fixes: 35326b61aecb ("bitops: add atomic bit operations in new API")
>>
>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> ---
> 
> I took a deep look into this.
> 
> Regardless of what the compiler bug is,
> 
> Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
> 

Thanks.

I'm far from sure it's a compiler bug. Just look at the base rate: how 
often does the code you just wrote fail because of a bug in your code, 
and how often is the root cause to be found in the compiler or the 
standard libraries.