lockless file descriptor lookup

[Jeff Roberson]

On Wed, 13 May 2009, John Baldwin wrote:

> On Monday 11 May 2009 11:32:17 pm Jeff Roberson wrote:
>> http://people.freebsd.org/~jeff/locklessfd.diff
>>
>> This patch implements a lockless lookup path for file descriptors.  The
>> meat of the algorithm is in fget_unlocked().  This returns a referenced
>> file descriptor, unlike fget_locked().  In the common case this reduces
>> the number of atomics required for fget() while allowing for lookups to
>> proceed concurrently with modifications to the table and preventing
>> preemption from causing context switches.
>
> Looks good.  My only comment would be to not remove the 'hold' comment
> completely from _fget(), but instead say that it always returns a refcount
> that must be dropped.  Basically:
>
> * The file's refcount will be bumped on return.  It should be dropped
> * with fdrop().
>
> or something like that in place of the old paragraph about the 'hold'
> parameter.

Yeah, I think I'll add a comment in the header too.  pho has tested it 
with some targeted tests and stress2 for a day or two.  I'm going to 
commit this evening so it gets as much exposure as possible before 
release.

Thanks,
Jeff

>
> -- 
> John Baldwin
>