Re: commit of [PATCH] Fix file lookup without ref

[Suzanne Wood]

 > 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 <linux-kernel@xxxxxxxxxxxxxxx>
 > > Date:       2006-04-19 17:00:12
 > > 
 > > commit ca99c1da080345e227cfb083c330a184d42e27f3
 > > tree e417b4c456ae31dc1dde8027b6be44a1a9f19395
 > > parent fb30d64568fd8f6a21afef987f11852a109723da
 > > author Dipankar Sarma <dipankar@xxxxxxxxxx> Wed, 19 Apr 2006 12:21:46 -0700
 > > committer Linus Torvalds <torvalds@xxxxxxxxxxx> Wed, 19 Apr 2006 23:13:51 -0700
 > > 
 > > [PATCH] Fix file lookup without ref
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