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Subjectbkl usage
Date
Hi. In my efforts to understand the linux kernel v2.4 I found the bkl being
used in kernel/acct.c to lock seemingly local data. Would someone please
explain what races this prevents vs. say:


--- linux/kernel/acct.c Mon Oct 30 01:02:56 2000
+++ linux-2.4.0-test10/kernel/acct.c Mon Oct 30 01:10:20 2000
@@ -41,6 +41,10 @@
* Oh, fsck... Oopsable SMP race in do_process_acct() - we must hold
* ->mmap_sem to walk the vma list of current->mm. Nasty, since it leaks
* a struct file opened for write. Fixed. 2/6/2000, AV.
+ *
+ * 2000-10-27 Modified by Nick <piggin@cyberone.com.au> to remove usage
+ * of the big kernel lock in favour of a local spinlock
+ *
*/

#include <linux/config.h>
@@ -77,6 +81,7 @@
static struct file *acct_file;
static struct timer_list acct_timer;
static void do_acct_process(long, struct file *);
+static spinlock_t acct_lock = SPIN_LOCK_UNLOCKED;

/*
* Called whenever the timer says to check the free space.
@@ -95,11 +100,11 @@
int res;
int act;

- lock_kernel();
+ spin_lock(&acct_lock);
res = acct_active;
if (!file || !acct_needcheck)
- goto out;
- unlock_kernel();
+ goto out_unlock;
+ spin_unlock(&acct_lock);

/* May block */
if (vfs_statfs(file->f_dentry->d_inode->i_sb, &sbuf))
@@ -113,14 +118,14 @@
act = 0;

/*
- * If some joker switched acct_file under us we'ld better be
+ * If some joker switched acct_file under us we'd better be
* silent and _not_ touch anything.
*/
- lock_kernel();
+ spin_lock(&acct_lock);
if (file != acct_file) {
if (act)
res = act>0;
- goto out;
+ goto out_unlock;
}

if (acct_active) {
@@ -140,8 +145,8 @@
acct_timer.expires = jiffies + ACCT_TIMEOUT*HZ;
add_timer(&acct_timer);
res = acct_active;
-out:
- unlock_kernel();
+out_unlock:
+ spin_unlock(&acct_lock);
return res;
}

@@ -182,7 +187,7 @@
}

error = 0;
- lock_kernel();
+ spin_lock(&acct_lock);
if (acct_file) {
old_acct = acct_file;
del_timer(&acct_timer);
@@ -200,7 +205,7 @@
acct_timer.expires = jiffies + ACCT_TIMEOUT*HZ;
add_timer(&acct_timer);
}
- unlock_kernel();
+ spin_unlock(&acct_lock);
if (old_acct) {
do_acct_process(0,old_acct);
filp_close(old_acct, NULL);
@@ -214,10 +219,24 @@

void acct_auto_close(kdev_t dev)
{
- lock_kernel();
- if (acct_file && acct_file->f_dentry->d_inode->i_dev == dev)
- sys_acct(NULL);
- unlock_kernel();
+ struct file *old_acct;
+
+ spin_lock(&acct_lock);
+ if (acct_file && acct_file->f_dentry->d_inode->i_dev == dev) {
+
+ /* Run the same code as sys_acct(NULL) here. This
simplifies locking */
+ old_acct = acct_file;
+ del_timer(&acct_timer);
+ acct_active = 0;
+ acct_needcheck = 0;
+ acct_file = NULL;
+
+ spin_unlock(&acct_lock);
+
+ do_acct_process(0, old_acct);
+ filp_close(old_acct, NULL);
+ } else
+ spin_unlock(&acct_lock);
}

/*
@@ -348,15 +367,15 @@
int acct_process(long exitcode)
{
struct file *file = NULL;
- lock_kernel();
+ spin_lock(&acct_lock);
if (acct_file) {
file = acct_file;
get_file(file);
- unlock_kernel();
+ spin_unlock(&acct_lock);
do_acct_process(exitcode, acct_file);
fput(file);
} else
- unlock_kernel();
+ spin_unlock(&acct_lock);
return 0;
}

-
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