Greg KH wrote:
> On Wed, Jul 03, 2002 at 11:26:27PM -0700, Dave Hansen wrote:
>>I saw your talk about driverfs at OLS and it got my attention.  When 
>>my BKL debugging patch showed some use of the BKL in driverfs, I was 
>>very dissapointed (you can blame Greg if you want).
> 
> Blame me?  Al Viro pushed that BKL into the file, not I :)

But you're so much closer :)  Did he push the mknod stuff too?

>>text from dmesg after BKL debugging patch:
>>release of recursive BKL hold, depth: 1
>>[ 0]main:492
>>[ 1]inode:149
> 
> This means what?

BKL was acquired at main.c:492 and current->lock_depth was 0
then
BKL was acquired at inode.c:149 and current->lock_depth was 1

>>I see no reason to hold the BKL in your situation.  I replaced it with 
>>i_sem in some places and just plain removed it in others.  I believe 
>>that you get all of the protection that you need from dcache_lock in 
>>the dentry insert and activate.  Can you prove me wrong?
> 
> I see no reason to really care :)
> Can you prove that driverfs (or pcihpfs or usbfs) accesses are on a
> critical path that removing the BKL usage here actually helps?

Nope.  I'm pretty sure that it isn't in a critical path anywhere, nor 
are there any performance benefits.  It is simply an annoying use that 
is relatively easy to remove.  It's kinda like using spaces instead of 
tabs; most people won't notice, but some people really care :)

> I think that driverfs_mknod() needs some kind of protection now that you
> have removed it.

Do you just want to make sure it isn't called concurrently, or is 
there some other BKL-protected area that you're concerned about. 
driverfs_mknod() doesn't appear to be doing anything sneaky like 
sleeping or calling itself, so I think a simple spinlock will work 
just fine.

> Um, you used spaces, please use tabs like the rest of the file, and how
> Documentation/CodingStyle mandates.

Arg.  I saw your talk twice so I really don't have an excuse.  fix 
attached.

-- 
Dave Hansen
haveblue@us.ibm.com