Re: [PATCH] remove BKL from driverfs

Dave Hansen
2002-07-04T07:24:02Z

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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

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--- linux-2.5.24-clean/fs/driverfs/inode.c Thu Jun 20 15:53:45 2002
+++ linux/fs/driverfs/inode.c Thu Jul 4 00:22:54 2002
@@ -128,6 +128,7 @@
return inode;
}

+static spinlock_t driverfs_mknod_lock = SPIN_LOCK_UNLOCKED;
static int driverfs_mknod(struct inode *dir, struct dentry *dentry, int mode, int dev)
{
struct inode *inode = driverfs_get_inode(dir->i_sb, mode, dev);
@@ -146,20 +147,20 @@
static int driverfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
{
int res;
- lock_kernel();
dentry->d_op = &driverfs_dentry_dir_ops;
+ spin_lock(&driverfs_mknod_lock);
res = driverfs_mknod(dir, dentry, mode | S_IFDIR, 0);
- unlock_kernel();
+ spin_unlock(&driverfs_mknod_lock);
return res;
}

static int driverfs_create(struct inode *dir, struct dentry *dentry, int mode)
{
int res;
- lock_kernel();
dentry->d_op = &driverfs_dentry_file_ops;
+ spin_lock(&driverfs_mknod_lock);
res = driverfs_mknod(dir, dentry, mode | S_IFREG, 0);
- unlock_kernel();
+ spin_unlock(&driverfs_mknod_lock);
return res;
}

@@ -211,9 +212,9 @@
if (driverfs_empty(dentry)) {
struct inode *inode = dentry->d_inode;

- lock_kernel();
+ down(&inode->i_sem);
inode->i_nlink--;
- unlock_kernel();
+ up(&inode->i_sem);
dput(dentry);
error = 0;
}
@@ -353,8 +354,9 @@
driverfs_file_lseek(struct file *file, loff_t offset, int orig)
{
loff_t retval = -EINVAL;
+ struct inode *inode = file->f_dentry->d_inode->i_mapping->host;

- lock_kernel();
+ down(&inode->i_sem);
switch(orig) {
case 0:
if (offset > 0) {
@@ -371,7 +373,7 @@
default:
break;
}
- unlock_kernel();
+ up(&inode->i_sem);
return retval;
}