Getting rid of atomic_load_acq_int(&fdp->fd_nfiles)) from fget_unlocked

[Konstantin Belousov]

On Sun, Jul 13, 2014 at 04:25:21PM +0300, Konstantin Belousov wrote:
> On Sun, Jul 13, 2014 at 05:55:00AM +0200, Mateusz Guzik wrote:
> > Currently:
> >         /*
> >          * Avoid reads reordering and then a first access to the
> >          * fdp->fd_ofiles table which could result in OOB operation.
> >          */
> >         if (fd < 0 || fd >= atomic_load_acq_int(&fdp->fd_nfiles))
> >                 return (EBADF);
> > 
> > However, if we put fd_nfiles and fd_otable into one atomically replaced
> > structure the only need to:
> > 1. make sure the pointer is read once
> > 2. issue a data dependency barrier - this is a noop on all supported
> > architectures and we don't even have approprate macro, so doing nothing
> > seems fine
> > 
> > The motivation is to boost performance to amortize for seqlock cost, in
> > case it hits the tree.
> > 
> > This has no impact on races with capability lookup.
> > 
> > In a microbenchmark of 16 threads reading from the same pipe fd
> > immediately returning EAGAIN the numbers are:
> > x vanilla-readpipe-run-sum             
> > + noacq-readpipe-run-sum
> > [..]
> >     N           Min           Max        Median           Avg        Stddev
> > x  20      13133671      14900364      13893331      13827075     471500.82
> > +  20      59479718      59527286      59496714      59499504     13752.968
> > Difference at 95.0% confidence
> > 	4.56724e+07 +/- 213483
> > 	330.312% +/- 1.54395%
> > 
> > There are 3 steps:
> > 1. tidy up capsicum to accept fde:
> > http://people.freebsd.org/~mjg/patches/single-fdtable-read-capsicum.patch
> > 2. add __READ_ONCE:
> > http://people.freebsd.org/~mjg/patches/read-once.patch
> > 3. put stuff into one structure:
> > http://people.freebsd.org/~mjg/patches/filedescenttable.patch
> > 
> > Comments?
> 
> We use 4-space indent for the continuation lines.  Look at the malloc(9)
> call in the patch 3.
> 
> The filedescenttable is really long name.  Could it be, for instance,
> fdescenttbl ?
> 
> Other than that, I think that the patches 2 and 3 are fine.  I did not
> looked at the patch 1.


As an afterthought, you do not need __READ_ONCE(), the __DEVOLATILE() alone
would do what you need as well.
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