Re: [RFC PATCH 1/2] locking: add mutex_lock_nospin()

[Waiman Long <longman@redhat.com>]

On 3/4/26 10:08 PM, Yafang Shao wrote:
> On Thu, Mar 5, 2026 at 11:00 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>> On Thu, 5 Mar 2026 10:33:00 +0800
>> Yafang Shao <laoar.shao@gmail.com> wrote:
>>
>>> Other tools may also read available_filter_functions, requiring each
>>> one to be patched individually to avoid this flaw—a clearly
>>> impractical solution.
>> What exactly is the issue?
> It makes no sense to spin unnecessarily when it can be avoided. We
> continuously improve the kernel to do the right thing—and unnecessary
> spinning is certainly not the right thing.
>
>> If a task does a while 1 in user space, it
>> wouldn't be much different.
> The while loop in user space performs actual work, whereas useless
> spinning does nothing but burn CPU cycles. My point is simple: if this
> unnecessary spinning isn't already considered an issue, it should
> be—it's something that clearly needs improvement.

The whole point of optimistic spinning is to reduce the lock acquisition 
latency. If the waiter sleeps, the unlock operation will have to wake up 
the waiter which can have a variable latency depending on how busy the 
system is at the time. Yes, it is burning CPU cycles while spinning, 
Most workloads will gain performance with this optimistic spinning 
feature. You do have a point that for system monitoring tools that 
observe the system behavior, they shouldn't burn that much CPU times 
that affect performance of real workload that the tools are monitoring.

BTW, you should expand the commit log of patch 1 to include the 
rationale of why we should add this feature to mutex as the information 
in the cover letter won't get included in the git log if this patch 
series is merged. You should also elaborate in comment on under what 
conditions should this this new mutex API be used.

Cheers,
Longman