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8ebc423238
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 <fweisbec@gmail.com> Reviewed-by: Ingo Molnar <mingo@elte.hu> Cc: Jeff Mahoney <jeffm@suse.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Bron Gondwana <brong@fastmail.fm> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Alexander Viro <viro@zeniv.linux.org.uk> LKML-Reference: <1239070789-13354-1-git-send-email-fweisbec@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
63 lines
1.9 KiB
C
63 lines
1.9 KiB
C
#include <linux/reiserfs_fs.h>
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#include <linux/mutex.h>
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/*
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* The previous reiserfs locking scheme was heavily based on
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* the tricky properties of the Bkl:
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*
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* - it was acquired recursively by a same task
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* - the performances relied on the release-while-schedule() property
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*
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* Now that we replace it by a mutex, we still want to keep the same
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* recursive property to avoid big changes in the code structure.
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* We use our own lock_owner here because the owner field on a mutex
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* is only available in SMP or mutex debugging, also we only need this field
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* for this mutex, no need for a system wide mutex facility.
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*
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* Also this lock is often released before a call that could block because
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* reiserfs performances were partialy based on the release while schedule()
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* property of the Bkl.
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*/
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void reiserfs_write_lock(struct super_block *s)
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{
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struct reiserfs_sb_info *sb_i = REISERFS_SB(s);
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if (sb_i->lock_owner != current) {
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mutex_lock(&sb_i->lock);
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sb_i->lock_owner = current;
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}
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/* No need to protect it, only the current task touches it */
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sb_i->lock_depth++;
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}
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void reiserfs_write_unlock(struct super_block *s)
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{
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struct reiserfs_sb_info *sb_i = REISERFS_SB(s);
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/*
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* Are we unlocking without even holding the lock?
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* Such a situation could even raise a BUG() if we don't
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* want the data become corrupted
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*/
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WARN_ONCE(sb_i->lock_owner != current,
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"Superblock write lock imbalance");
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if (--sb_i->lock_depth == -1) {
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sb_i->lock_owner = NULL;
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mutex_unlock(&sb_i->lock);
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}
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}
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/*
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* Utility function to force a BUG if it is called without the superblock
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* write lock held. caller is the string printed just before calling BUG()
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*/
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void reiserfs_check_lock_depth(struct super_block *sb, char *caller)
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{
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struct reiserfs_sb_info *sb_i = REISERFS_SB(sb);
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if (sb_i->lock_depth < 0)
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reiserfs_panic(sb, "%s called without kernel lock held %d",
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caller);
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}
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