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SubjectRe: spinlock contention of files->file_lock
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On Tue, 2013-10-01 at 04:27 +0100, Al Viro wrote:
> On Mon, Sep 30, 2013 at 07:02:23PM -0700, Linus Torvalds wrote:
>
> > Shouldn't a cmpxchg() in just the dup2 code solve that?
> >
> > If the old value was NULL, you'd have to repeat and go back and see if
> > the open_fds[] bit had been cleared in the meantime (ie it's NULL not
> > because somebody else is busy installing it, but because somebody just
> > uninstalled it).
>
> Yechh... Under ->file_lock (in do_dup2()), hopefully? Or you'll get
> all kinds of fun with close() thrown into the game, as well...
>
> > But yeah, I do agree that that sounds nasty and a complication I
> > hadn't even thought about. dup2() does violate our normal "let's
> > pre-allocate the fd slot" rule. Ugh.
>
> Hell knows... Descriptor handling *is* pretty well isolated these
> days, so it just might be doable without disrupting the living hell
> out of anything else. fs/file.c is pretty much it - everything else
> goes through it.

I have a patch mostly working here, and pretty short.

I'll do proper tests before posting it tomorrow.

fs/fcntl.c | 7 ++-----
fs/file.c | 17 +++++++++++++----
fs/open.c | 21 +++++++++++++++++----
include/linux/fdtable.h | 1 +
4 files changed, 33 insertions(+), 13 deletions(-)

Thanks !




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