> From Suzanne Wood Tue May 9 23:53:03 2006
> In studying proc_readfd() of fs/proc/base.c, I'd looked back
> at the linux-2.5.60 version which was prior to the conversion
> to RCU and noticed that rather than straight spin_lock() as
> introduced in this patch, proc_fd_link() and proc_lookupfd()
> used read_lock(&files->file_lock). Similarly, for __do_SAK()
> in drivers/char/tty_io.c
In include/linux/file.h, a change between linux-2.5.60 and
linux-2.6.17 was rwlock_t file_lock to spinlock_t file_lock.
In http://marc.theaimsgroup.com/?l=lse-tech&m=98235007317770&w=2
John Hawkes apparently explains this in Finding #3,
"Because clone'd threads share the identical files_struct, we have
hundreds of threads doing the read_lock() on the same
(rwlock_t)file_lock. This does not cause overt lock contention (because
with this test load there is no writer-owner of the file_lock), but it
does mean that each read_lock() and each read_unlock() dirties the
file_lock word, which produces that cacheblock ping-pong effect when
another thread on another CPU accesses that same file_lock word."
as referenced by
http://lwn.net/2001/0412/a/fd-management.php3
Thanks.
Suzanne
> > List: git-commits-head
> > Subject: [PATCH] Fix file lookup without ref
> > From: Linux Kernel Mailing List <[email protected]>
> > Date: 2006-04-19 17:00:12
> >
> > commit ca99c1da080345e227cfb083c330a184d42e27f3
> > tree e417b4c456ae31dc1dde8027b6be44a1a9f19395
> > parent fb30d64568fd8f6a21afef987f11852a109723da
> > author Dipankar Sarma <[email protected]> Wed, 19 Apr 2006 12:21:46 -0700
> > committer Linus Torvalds <[email protected]> Wed, 19 Apr 2006 23:13:51 -0700
> >
> > [PATCH] Fix file lookup without ref
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