Re: [PATCH] mm: Make failslab writable again

[Alexander Atanasov <alexander.atanasov@virtuozzo.com>]

On 20.09.22 12:29, Vlastimil Babka wrote:
> On 9/20/22 11:17, Alexander Atanasov wrote:
>> Hello,
>>
>> On 20.09.22 11:42, Vlastimil Babka wrote:
>>>> +static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
>>>> +                size_t length)
>>>> +{
>>>> +    if (s->refcount > 1)
>>>> +        return -EINVAL;
>>>> +
>>>> +    s->flags &= ~SLAB_FAILSLAB;
>>>> +    if (buf[0] == '1')
>>>> +        s->flags |= SLAB_FAILSLAB;
>>>
>>> Could we at least use a temporary variable to set up the final value and
>>> then do a WRITE_ONCE() to s->flags, so the compiler is not allowed to do
>>> some funky stuff? Assuming this is really the only place where we modify
>>> s->flags during runtime, so we can't miss other updates due to RMW.
>>
>> Since it is set or clear - instead of temporary variable and potentially two
>> writes and RMW issues i would suggest this:
>> +    if (buf[0] == '1')
>> +        s->flags |= SLAB_FAILSLAB;
>> +       else
>> +        s->flags &= ~SLAB_FAILSLAB;
> 
> This way also has RMW issues, and also the compiler is allowed to
> temporarily modify s->flags any way it likes; with WRITE_ONCE() it can't.

Okay, so the safest way is this?

if (buf[0] == '1')
	WRITE_ONCE(s->flags, READ_ONCE(s->flags) | SLAB_FAILSLAB);
else
	WRITE_ONCE(s->flags, READ_ONCE(s->flags) & ~SLAB_FAILSLAB);

It got me thinking how many places would break if the compiler
starts to temporariliy modify the flags - i hope it never does.

-- 
Regards,
Alexander Atanasov