Re: [PATCH] jiffies: Cast to unsigned long for secs_to_jiffies() conversion

[Easwar Hariharan <eahariha@linux.microsoft.com>]

On 1/31/2025 12:10 AM, Geert Uytterhoeven wrote:
> Hi Jiri,
> 
> CC linux-xfs
> 
> On Fri, 31 Jan 2025 at 08:05, Jiri Slaby <jirislaby@kernel.org> wrote:
>> On 30. 01. 25, 21:14, David Laight wrote:
>>> On Thu, 30 Jan 2025 18:43:17 +0000
>>> Easwar Hariharan <eahariha@linux.microsoft.com> wrote:
>>>
>>>> While converting users of msecs_to_jiffies(), lkp reported that some
>>>> range checks would always be true because of the mismatch between the
>>>> implied int value of secs_to_jiffies() vs the unsigned long
>>>> return value of the msecs_to_jiffies() calls it was replacing. Fix this
>>>> by casting secs_to_jiffies() values as unsigned long.
>>>
>>> Surely 'unsigned long' can't be the right type ?
>>> It changes between 32bit and 64bit systems.
>>> Either it is allowed to wrap - so should be 32bit on both,
>>> or wrapping is unexpected and it needs to be 64bit on both.
>>
>> But jiffies are really ulong.
> 
> That's a good reason to make the change.
> E.g. msecs_to_jiffies() does return unsigned long.
> 
> Note that this change may cause fall-out, e.g.
> 
>     int val = 5.
> 
>     pr_debug("timeout = %u jiffies\n", secs_to_jiffies(val));
>                         ^^
>                         must be changed to %lu
> 
> More importantly, I doubt this change is guaranteed to fix the
> reported issue.  The code[*] in retry_timeout_seconds_store() does:
> 
>     int val;
>     ...
>     if (val < -1 || val > 86400)
>             return -EINVAL;
>     ...
>     if (val != -1)
>             ASSERT(secs_to_jiffies(val) < LONG_MAX);
> 
> As HZ is a known (rather small) constant, and val is range-checked
> before, the compiler can still devise that the condition is always true.
> So I think that assertion should just be removed.
> 
> [*] Before commit b524e0335da22473 ("xfs: convert timeouts to
>     secs_to_jiffies()"), which was applied to the MM tree only 3
>     days ago, the code used msecs_to_jiffies() * MSEC_PER_SEC,
>     which is more complex than a simple multiplication, and harder for
>     the compiler to analyze statically, thus not triggering the warning
>     that easily...
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 

Thanks, Jiri and Geert. Geert, am I correct in understanding you that
you're suggesting v2 of the series[1] to convert msecs_to_jiffies()
calls to secs_to_jiffies() remove the ASSERT as redundant, while also
keeping this patch because ulong is the right type for jiffies?

[1]
https://lore.kernel.org/all/20250128-converge-secs-to-jiffies-part-two-v1-0-9a6ecf0b2308@linux.microsoft.com/

Thanks,
Easwar