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Date:      Wed, 13 Aug 2014 03:00:46 +0200
From:      Mateusz Guzik <mjguzik@gmail.com>
To:        Konstantin Belousov <kostikbel@gmail.com>
Cc:        freebsd-arch@freebsd.org
Subject:   Re: Getting rid of atomic_load_acq_int(&fdp->fd_nfiles)) from fget_unlocked
Message-ID:  <20140813010046.GB17869@dft-labs.eu>
In-Reply-To: <20140713133421.GA93733@kib.kiev.ua>
References:  <20140713035500.GC16884@dft-labs.eu> <20140713132521.GY93733@kib.kiev.ua> <20140713133421.GA93733@kib.kiev.ua>

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On Sun, Jul 13, 2014 at 04:34:21PM +0300, Konstantin Belousov wrote:
> 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.

Turns out patch 2 was quite bad.

Reading http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf
(pdf page 77) reveals:
A cast of a value to a qualified type has no effect; the qualification
(volatile, say) can have no effect on the access since it has occurred
prior to the cast. If it is necessary to access a non-volatile object
using volatile semantics, the technique is to cast the address of the
object to the appropriate pointer-to-qualified type, then dereference
that pointer. 

So how about we just follow the recomandation and also get the type
automagically like linux folks do (added to sys/param.h):
#define READ_ONCE(var)  (*(volatile __typeof(var) *)&(var))

http://people.freebsd.org/~mjg/patches/read-once.patch

I incorporated suggested changes have overwritten old patches.
http://people.freebsd.org/~mjg/patches/filedescenttable.patch

I would like to commit these changes this week with 2 weeks mfc.

-- 
Mateusz Guzik <mjguzik gmail.com>



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