lkml.org 
[lkml]   [2014]   [Oct]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: semantics of reader/writer semaphores in rt patch
On Sun, Oct 26, 2014 at 12:21:31AM +0200, Thomas Gleixner wrote:
> On Sun, 26 Oct 2014, Thomas Gleixner wrote:
> > On Thu, 23 Oct 2014, Chris Friesen wrote:
> > > Does the RT kernel just disallow this sort of algorithm?
> >
> > Yes. For a good reason. Let's add thread C
> >
> > A B C
> > down_read(X)
> > down_write(X)
> > lock(Y)
> > modify data
> > unlock(Y)
> > wake(B)
> > down_read(X)
> >
> > Due to the mainline rwsem fairness semantics:
> >
> > A holds X, C is blocked on A and B is blocked on A.
> >
> > Deadlock, without RT and the single reader restriction being involved.
> >
> > So RT does not violate ANY of the existing mainline semantics, it just
> > imposes a performance impact of not allowing multiple readers.
>
> @peterz: It might be worthwhile to have a CONFIG_LOCKDEP=y dependent
> mode which restricts concurrent readers to 1 in mainline to catch this
> kind of stuff. Hmm?

There were patches by ego that fix lockdep's read side tracking. I need
to find a few spare days to look at those :/


\
 
 \ /
  Last update: 2014-10-26 09:21    [W:0.088 / U:0.240 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site