Re: [PATCH] staging: axis-fifo: minimize lock duration in write

[Josh Law <hlcj1234567@gmail.com>]

9 Mar 2026 16:56:37 Greg Kroah-Hartman <gregkh@linuxfoundation.org>:

> On Mon, Mar 09, 2026 at 04:11:25PM +0000, Josh Law wrote:
>> 9 Mar 2026 16:07:50 Greg Kroah-Hartman <gregkh@linuxfoundation.org>:
>>
>>> On Sun, Mar 01, 2026 at 12:58:36AM +0000, Josh Law wrote:
>>>> Memory allocation and copy from user space with vmemdup_user() is relatively slow and can sleep. Move it outside the lock to minimize the time the mutex is held, reducing contention for concurrent accesses.
>>>
>>> Something happened with your line wrapping :(
>>>
>>> And are you sure that your change works?  What is wrong with grabbing
>>> the lock this way?
>>>
>>> thanks,
>>>
>>> greg k-h
>>
>> The issue with grabbing the lock before vmemdup_user() is that the allocation and user-space copy can take a significant amount of time and can sleep.
>
> But what is wrong with that?
>
>> Also, it's just to be more efficient haha, and yes I'm sure this change works, I own the hardware, and it operates better...
>
> Great, please put the performance numbers in the patch, that will show
> the need for this change.
>
> thanks,
>
> greg k-h

Hi Greg,
Fair point. Because mutexes are allowed to sleep, holding it during vmemdup_user() isn't inherently a bug. The issue is that while it sleeps, it blocks other processes from acquiring the lock, which bottlenecks CPU usage times slightly



Perf numbers:

sys before: about 18 seconds
sys after: 14 seconds

V/R

Josh law