From 8ebc423238341b52912c7295b045a32477b33f09 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 7 Apr 2009 04:19:49 +0200 Subject: reiserfs: kill-the-BKL This patch is an attempt to remove the Bkl based locking scheme from reiserfs and is intended. It is a bit inspired from an old attempt by Peter Zijlstra: http://lkml.indiana.edu/hypermail/linux/kernel/0704.2/2174.html The bkl is heavily used in this filesystem to prevent from concurrent write accesses on the filesystem. Reiserfs makes a deep use of the specific properties of the Bkl: - It can be acqquired recursively by a same task - It is released on the schedule() calls and reacquired when schedule() returns The two properties above are a roadmap for the reiserfs write locking so it's very hard to simply replace it with a common mutex. - We need a recursive-able locking unless we want to restructure several blocks of the code. - We need to identify the sites where the bkl was implictly relaxed (schedule, wait, sync, etc...) so that we can in turn release and reacquire our new lock explicitly. Such implicit releases of the lock are often required to let other resources producer/consumer do their job or we can suffer unexpected starvations or deadlocks. So the new lock that replaces the bkl here is a per superblock mutex with a specific property: it can be acquired recursively by a same task, like the bkl. For such purpose, we integrate a lock owner and a lock depth field on the superblock information structure. The first axis on this patch is to turn reiserfs_write_(un)lock() function into a wrapper to manage this mutex. Also some explicit calls to lock_kernel() have been converted to reiserfs_write_lock() helpers. The second axis is to find the important blocking sites (schedule...(), wait_on_buffer(), sync_dirty_buffer(), etc...) and then apply an explicit release of the write lock on these locations before blocking. Then we can safely wait for those who can give us resources or those who need some. Typically this is a fight between the current writer, the reiserfs workqueue (aka the async commiter) and the pdflush threads. The third axis is a consequence of the second. The write lock is usually on top of a lock dependency chain which can include the journal lock, the flush lock or the commit lock. So it's dangerous to release and trying to reacquire the write lock while we still hold other locks. This is fine with the bkl: T1 T2 lock_kernel() mutex_lock(A) unlock_kernel() // do something lock_kernel() mutex_lock(A) -> already locked by T1 schedule() (and then unlock_kernel()) lock_kernel() mutex_unlock(A) .... This is not fine with a mutex: T1 T2 mutex_lock(write) mutex_lock(A) mutex_unlock(write) // do something mutex_lock(write) mutex_lock(A) -> already locked by T1 schedule() mutex_lock(write) -> already locked by T2 deadlock The solution in this patch is to provide a helper which releases the write lock and sleep a bit if we can't lock a mutex that depend on it. It's another simulation of the bkl behaviour. The last axis is to locate the fs callbacks that are called with the bkl held, according to Documentation/filesystem/Locking. Those are: - reiserfs_remount - reiserfs_fill_super - reiserfs_put_super Reiserfs didn't need to explicitly lock because of the context of these callbacks. But now we must take care of that with the new locking. After this patch, reiserfs suffers from a slight performance regression (for now). On UP, a high volume write with dd reports an average of 27 MB/s instead of 30 MB/s without the patch applied. Signed-off-by: Frederic Weisbecker Reviewed-by: Ingo Molnar Cc: Jeff Mahoney Cc: Peter Zijlstra Cc: Bron Gondwana Cc: Andrew Morton Cc: Linus Torvalds Cc: Alexander Viro LKML-Reference: <1239070789-13354-1-git-send-email-fweisbec@gmail.com> Signed-off-by: Ingo Molnar --- fs/reiserfs/super.c | 37 ++++++++++++++++++++++++++++++------- 1 file changed, 30 insertions(+), 7 deletions(-) (limited to 'fs/reiserfs/super.c') diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c index 7adea74d6a8..e1cfb80d0bf 100644 --- a/fs/reiserfs/super.c +++ b/fs/reiserfs/super.c @@ -465,7 +465,7 @@ static void reiserfs_put_super(struct super_block *s) struct reiserfs_transaction_handle th; th.t_trans_id = 0; - lock_kernel(); + reiserfs_write_lock(s); if (s->s_dirt) reiserfs_write_super(s); @@ -499,10 +499,10 @@ static void reiserfs_put_super(struct super_block *s) reiserfs_proc_info_done(s); + reiserfs_write_unlock(s); + mutex_destroy(&REISERFS_SB(s)->lock); kfree(s->s_fs_info); s->s_fs_info = NULL; - - unlock_kernel(); } static struct kmem_cache *reiserfs_inode_cachep; @@ -1168,11 +1168,14 @@ static int reiserfs_remount(struct super_block *s, int *mount_flags, char *arg) unsigned int qfmt = 0; #ifdef CONFIG_QUOTA int i; +#endif + + reiserfs_write_lock(s); +#ifdef CONFIG_QUOTA memcpy(qf_names, REISERFS_SB(s)->s_qf_names, sizeof(qf_names)); #endif - lock_kernel(); rs = SB_DISK_SUPER_BLOCK(s); if (!reiserfs_parse_options @@ -1295,12 +1298,12 @@ static int reiserfs_remount(struct super_block *s, int *mount_flags, char *arg) out_ok: replace_mount_options(s, new_opts); - unlock_kernel(); + reiserfs_write_unlock(s); return 0; out_err: kfree(new_opts); - unlock_kernel(); + reiserfs_write_unlock(s); return err; } @@ -1404,7 +1407,9 @@ static int read_super_block(struct super_block *s, int offset) static int reread_meta_blocks(struct super_block *s) { ll_rw_block(READ, 1, &(SB_BUFFER_WITH_SB(s))); + reiserfs_write_unlock(s); wait_on_buffer(SB_BUFFER_WITH_SB(s)); + reiserfs_write_lock(s); if (!buffer_uptodate(SB_BUFFER_WITH_SB(s))) { reiserfs_warning(s, "reiserfs-2504", "error reading the super"); return 1; @@ -1613,7 +1618,7 @@ static int reiserfs_fill_super(struct super_block *s, void *data, int silent) sbi = kzalloc(sizeof(struct reiserfs_sb_info), GFP_KERNEL); if (!sbi) { errval = -ENOMEM; - goto error; + goto error_alloc; } s->s_fs_info = sbi; /* Set default values for options: non-aggressive tails, RO on errors */ @@ -1627,6 +1632,20 @@ static int reiserfs_fill_super(struct super_block *s, void *data, int silent) /* setup default block allocator options */ reiserfs_init_alloc_options(s); + mutex_init(&REISERFS_SB(s)->lock); + REISERFS_SB(s)->lock_depth = -1; + + /* + * This function is called with the bkl, which also was the old + * locking used here. + * do_journal_begin() will soon check if we hold the lock (ie: was the + * bkl). This is likely because do_journal_begin() has several another + * callers because at this time, it doesn't seem to be necessary to + * protect against anything. + * Anyway, let's be conservative and lock for now. + */ + reiserfs_write_lock(s); + jdev_name = NULL; if (reiserfs_parse_options (s, (char *)data, &(sbi->s_mount_opt), &blocks, &jdev_name, @@ -1852,9 +1871,13 @@ static int reiserfs_fill_super(struct super_block *s, void *data, int silent) init_waitqueue_head(&(sbi->s_wait)); spin_lock_init(&sbi->bitmap_lock); + reiserfs_write_unlock(s); + return (0); error: + reiserfs_write_unlock(s); +error_alloc: if (jinit_done) { /* kill the commit thread, free journal ram */ journal_release_error(NULL, s); } -- cgit v1.2.3-18-g5258