linux-kernel - Re: [PATCH] kernfs: switch global kernfs

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Date:   Mon, 22 Nov 2021 09:39:28 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Tejun Heo <tj@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock

On Thu, Nov 18, 2021 at 01:03:32PM -1000, Tejun Heo wrote:
> 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@...nel.org>
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> 
> Acked-by: Tejun Heo <tj@...nel.org>
> 
> 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.

Greg, Do you mind picking this patch?