Re: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock

[Tejun Heo]

On Thu, Nov 18, 2021 at 03:00:08PM -0800, Minchan Kim wrote:
> The kernfs implementation has big lock granularity(kernfs_rwsem) so
> every kernfs-based(e.g., sysfs, cgroup) fs are able to compete the
> lock. It makes trouble for some cases to wait the global lock
> for a long time even though they are totally independent contexts
> each other.
> 
> A general example is process A goes under direct reclaim with holding
> the lock when it accessed the file in sysfs and process B is waiting
> the lock with exclusive mode and then process C is waiting the lock
> until process B could finish the job after it gets the lock from
> process A.
> 
> This patch switches the global kernfs_rwsem to per-fs lock, which
> put the rwsem into kernfs_root.
> 
> Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Greg, I think this is the right thing to do even if there is no concrete
performance argument (not saying there isn't). It's just weird to entangle
these completely unrelated users in a single rwsem.

Thanks.

-- 
tejun