Re: [RFC v4] mm/damon: add synchronous commit for commit_inputs

[SeongJae Park <sj@kernel.org>]

On Mon, 23 Mar 2026 22:19:55 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> Hi SeongJae,
> 
> Follow-up on my previous email regarding the locking concerns.
> 
> > I admit I don't have an elegant solution yet. Here are my current
> > thought:
> > 
> > Add a timeout to wait_for_completion() (e.g.,
> > wait_for_completion_timeout()), using 'damos_watermarks.interval' as the
> > upper bound. This prevents indefinite blocking of 'param_lock', though
> > it still holds the global lock for up to several seconds.
> 
> After further analysis, I realized that since both 'commit_inputs' and
> 'enabled' module parameters are protected by the global 'param_lock',
> stopping kdamond gracefully (via enabled=N) is serialized with
> 'commit_inputs' writes. Therefore, a classic deadlock scenario should
> not occur in normal operation.
> 
> However, I'm considering edge cases where kdamond might terminate
> unexpectedly. In such cases, commit_inputs_store() could hold
> 'param_lock' indefinitely, blocking other module parameter updates
> system-wide.
> 
> To mitigate this risk defensively, would you accept adding a timeout to
> wait_for_completion()? This ensures 'param_lock' is eventually released
> even if kdamond fails unexpectedly.
> 
> Please let me know your preference. :>

As I mentioned on the reply to the original mail of this mail, I think such
infinite wait will not happen because damon_call() is aware of the kdamond
stop, and therefore the timeout is not needed.  Please double check and let me
know if I'm missing something, though, as a reply to my reply.


Thanks,
SJ

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