Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Bernd Schubert]

On 9/19/23 08:11, Kemeng Shi wrote:
> on 9/16/2023 7:06 PM, Bernd Schubert wrote:
> > On 9/14/23 17:45, Kemeng Shi wrote:
> > > Commit 670d21c6e17f6 ("fuse: remove reliance on bdi congestion") change how
> > > congestion_threshold is used and lock in
> > > fuse_conn_congestion_threshold_write is not needed anymore.
> > > 1. Access to supe_block is removed along with removing of bdi congestion.
> > > Then down_read(&fc->killsb) which protecting access to super_block is no
> > > needed.
> > > 2. Compare num_background and congestion_threshold without holding
> > > bg_lock. Then there is no need to hold bg_lock to update
> > > congestion_threshold.
> > >
> > > Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> > > ---
> > >    fs/fuse/control.c | 4 ----
> > >    1 file changed, 4 deletions(-)
> > >
> > > diff --git a/fs/fuse/control.c b/fs/fuse/control.c
> > > index 247ef4f76761..c5d7bf80efed 100644
> > > --- a/fs/fuse/control.c
> > > +++ b/fs/fuse/control.c
> > > @@ -174,11 +174,7 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
> > >        if (!fc)
> > >            goto out;
> > >    -    down_read(&fc->killsb);
> > > -    spin_lock(&fc->bg_lock);
> > >        fc->congestion_threshold = val;
> > > -    spin_unlock(&fc->bg_lock);
> > > -    up_read(&fc->killsb);
> > >        fuse_conn_put(fc);
> > >    out:
> > >        return ret;
> >
> > Yeah, I don't see readers holding any of these locks.
> > I just wonder if it wouldn't be better to use WRITE_ONCE to ensure a single atomic operation to store the value.
> Sure, WRITE_ONCE looks better. I wonder if we should use READ_ONCE from reader.
> Would like to get any advice. Thanks!

I'm not entirely sure either, but I _think_ the compiler is free to 
store a 32 bit value  with multiple operations (like 2 x 16 bit). In 
that case a competing reading thread might read garbage...
Although I don't see this documented here
https://www.kernel.org/doc/Documentation/memory-barriers.txt
Though documented there is that the compile is free to optimize out the 
storage at all, see
"(*) Similarly, the compiler is within its rights to omit a store entirely"


Regarding READ_ONCE, I don't have a strong opinion, if the compiler 
makes some optimizations and the value would be wrong for a few cycles, 
would that matter for that variable? Unless the compiler would be really 
creative and the variable would get never updated... For sure READ_ONCE 
would be safer, but I don't know if it is needed
SSee section
"The compiler is within its rights to omit a load entirely if it know"
in the document above.

Thanks,
Bernd