2.5.6-pre3 Fix small race in BSD accounting [part 2]
PATCH 1/1 2.5.6-pre3 Fix small race in BSD accounting [part 2]
Linus,
While looking at the bug fix for part 1 I coded up this patch to change the BSD accounting code to use a spinlock instead of the BKL.
This patch assumes that part 1 has been applied.
Signature · 166 lines
Bob Miller Email: [email protected] Open Source Development Lab Phone: 503.626.2455 Ext. 17 diff -ur linux-2.5.6-orig/kernel/acct.c linux-2.5.6/kernel/acct.c --- linux-2.5.6-orig/kernel/acct.c Mon Mar 11 11:19:58 2002 +++ linux-2.5.6/kernel/acct.c Mon Mar 11 11:33:14 2002 @@ -72,14 +72,30 @@ /* * External references and all of the globals. */ - -static volatile int acct_active; -static volatile int acct_needcheck; -static struct file *acct_file; -static struct timer_list acct_timer; static void do_acct_process(long, struct file *); /* + * This structure is used so that all the data protected by lock + * can be placed in the same cache line as the lock. This primes + * the cache line to have the data after getting the lock. + */ +struct acct_glbs { + spinlock_t lock; + volatile int active; + volatile int needcheck; + struct file *file; + struct timer_list timer; +}; + +static struct acct_glbs acct_globals __cacheline_aligned = {SPIN_LOCK_UNLOCKED}; + +#define acct_lock acct_globals.lock +#define acct_active acct_globals.active +#define acct_needcheck acct_globals.needcheck +#define acct_file acct_globals.file +#define acct_timer acct_globals.timer + +/* * Called whenever the timer says to check the free space. */ static void acct_timeout(unsigned long unused) @@ -96,11 +112,11 @@ int res; int act; - lock_kernel(); + spin_lock(&acct_lock); res = acct_active; if (!file || !acct_needcheck) goto out; - unlock_kernel(); + spin_unlock(&acct_lock); /* May block */ if (vfs_statfs(file->f_dentry->d_inode->i_sb, &sbuf)) @@ -117,7 +133,7 @@ * If some joker switched acct_file under us we'ld better be * silent and _not_ touch anything. */ - lock_kernel(); + spin_lock(&acct_lock); if (file != acct_file) { if (act) res = act>0; @@ -142,22 +158,26 @@ add_timer(&acct_timer); res = acct_active; out: - unlock_kernel(); + spin_unlock(&acct_lock); return res; } /* - * sys_acct() is the only system call needed to implement process - * accounting. It takes the name of the file where accounting records - * should be written. If the filename is NULL, accounting will be - * shutdown. + * acct_common() is the main routine that implements process accounting. + * It takes the name of the file where accounting records should be + * written. If the filename is NULL, accounting will be shutdown. */ -asmlinkage long sys_acct(const char *name) +long acct_common(const char *name, int locked) { struct file *file = NULL, *old_acct = NULL; char *tmp; int error; + /* + * Should only have locked set when name is NULL (enforce this). + */ + BUG_ON(locked && name); + if (!capable(CAP_SYS_PACCT)) return -EPERM; @@ -183,7 +203,8 @@ } error = 0; - lock_kernel(); + if (!locked) + spin_lock(&acct_lock); if (acct_file) { old_acct = acct_file; del_timer(&acct_timer); @@ -201,7 +222,7 @@ 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); @@ -213,12 +234,25 @@ goto out; } +/* + * sys_acct() is the only system call needed to implement process + * accounting. It takes the name of the file where accounting records + * should be written. If the filename is NULL, accounting will be + * shutdown. + */ +asmlinkage long sys_acct(const char *name) +{ + return (acct_common(name, 0)); +} + void acct_auto_close(struct super_block *sb) { - lock_kernel(); - if (acct_file && acct_file->f_dentry->d_inode->i_sb == sb) - sys_acct(NULL); - unlock_kernel(); + spin_lock(&acct_lock); + if (acct_file && acct_file->f_dentry->d_inode->i_sb == sb) { + (void) acct_common(NULL, 1); + } else { + spin_unlock(&acct_lock); + } } /* @@ -349,15 +383,15 @@ 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, file); fput(file); } else - unlock_kernel(); + spin_unlock(&acct_lock); return 0; }
Quoted 3 lines · Bob Miller
While looking at the bug fix for part 1 I coded up this patch to change the BSD accounting code to use a spinlock instead of the BKL.
Oh, Good Job - BKL is evil. And I think that is partly evident in the resulting code, and I have a couple comments about it.
I suspect the recursive nature of the BKL (and perhaps the locking rules such that you don't always hold alock, i.e. if name is not NULL) are responsible for:
if (!locked) spin_lock(&acct_lock);
which really isn't the prettiest or safest thing, although I don't actually see any problems with it here. With the BKL removed, it may be better to rewrite the code in a cleaner and saner way.
I'd much rather see sane locking rules where we knew the callers and each function entry clearly either held or does not hold the spin_lock. Make sure we don't call anything holding the lock, et cetera ...
Also, I like the struct but the defines are a bit ugly. Why not just s/acct_lock/acct_globals.lock/, for example, in the code? Or Just call the instance of the struct "acct" and have acct.lock, etc.
In other words, good job, but this is a development kernel - rip some of this cruft up and make it perfect, no?
Robert Love
Quoted 24 lines · Robert Love
Quoted 3 lines · Bob Miller
While looking at the bug fix for part 1 I coded up this patch to change the BSD accounting code to use a spinlock instead of the BKL.
Oh, Good Job - BKL is evil. And I think that is partly evident in the resulting code, and I have a couple comments about it.
I suspect the recursive nature of the BKL (and perhaps the locking rules such that you don't always hold alock, i.e. if name is not NULL) are responsible for:
if (!locked) spin_lock(&acct_lock);which really isn't the prettiest or safest thing, although I don't actually see any problems with it here. With the BKL removed, it may be better to rewrite the code in a cleaner and saner way.
I'd much rather see sane locking rules where we knew the callers and each function entry clearly either held or does not hold the
spin_lock. Make sure we don't call anything holding the lock, et cetera ...
Quoted 8 lines
Also, I like the struct but the defines are a bit ugly. Why not just s/acct_lock/acct_globals.lock/, for example, in the code? Or Just call the instance of the struct "acct" and have acct.lock, etc.
In other words, good job, but this is a development kernel - rip some of this cruft up and make it perfect, no?
Robert Love
Robert,
Thanks for the comments. I was going for the minimal diff approach, but I think the code would be better with a little re-arranging. I'll make a cut at it and re-submit.
Signature · 2 lines
Bob Miller Email: [email protected] Open Source Development Lab Phone: 503.626.2455 Ext. 17