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

[John Baldwin]

On Saturday, July 12, 2014 11:55:00 pm 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

The KASSERT() on the 'fd' being valid was lost from cap_fcntl_check().  Should 
probably put that back.  However, cap_fcntl_check() is now no longer used.  
Instead, you should just rename cap_fcntl_check_fd() to cap_fcntl_check() and 
remove the old one.

> 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

Anotehr name besides the one suggested by Konstantin could just be 'struct 
filetable'.  Even shorter while not having abbreviations.  Also, I think you 
can leave the comment in 'struct filedesc' as just 'open files' in that case.

-- 
John Baldwin