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linux/fs/btrfs
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Josef Bacik c9ea7b24ce Btrfs: stop caching thread if extent_commit_sem is contended 
We can starve out the transaction commit with a bunch of caching threads all
running at the same time.  This is because we will only drop the
extent_commit_sem if we need_resched(), which isn't likely to happen since we
will be reading a lot from the disk so have already schedule()'ed plenty.  Alex
observed that he could starve out a transaction commit for up to a minute with
32 caching threads all running at once.  This will allow us to drop the
extent_commit_sem to allow the transaction commit to swap the commit_root out
and then all the cachers will start back up. Here is an explanation provided by
Igno

So, just to fill in what happens in this loop:

                                mutex_unlock(&caching_ctl->mutex);
                                cond_resched();
                                goto again;

where 'again:' takes caching_ctl->mutex and fs_info->extent_commit_sem
again:

        again:
                mutex_lock(&caching_ctl->mutex);
                /* need to make sure the commit_root doesn't disappear */
                down_read(&fs_info->extent_commit_sem);

So, if I'm reading the code correct, there can be a fair amount of
concurrency here: there may be multiple 'caching kthreads' per filesystem
active, while there's one fs_info->extent_commit_sem per filesystem
AFAICS.

So, what happens if there are a lot of CPUs all busy holding the
->extent_commit_sem rwsem read-locked and a writer arrives? They'd all
rush to try to release the fs_info->extent_commit_sem, and they'd block in
the down_read() because there's a writer waiting.

So there's a guarantee of forward progress. This should answer akpm's
concern I think.

Thanks,

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>
2014-01-28 13:20:10 -08:00
..
tests Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
acl.c btrfs: remove dead code 2014-01-28 13:19:50 -08:00
async-thread.c Btrfs: fix __btrfs_start_workers retval 2013-11-20 20:42:11 -05:00
async-thread.h Btrfs: eliminate races in worker stopping code 2013-10-04 16:02:13 -04:00
backref.c Btrfs: fix deadlock when iterating inode refs and running delayed inodes 2014-01-28 13:19:59 -08:00
backref.h Btrfs: allocate prelim_ref with a slab allocater 2013-09-01 08:16:27 -04:00
btrfs_inode.h Btrfs: introduce the delayed inode ref deletion for the single link inode 2014-01-28 13:20:09 -08:00
check-integrity.c Btrfs: fix check-integrity to look at the referenced data properly 2014-01-28 13:19:21 -08:00
check-integrity.h block: submit_bio_wait() conversions 2013-11-24 16:33:41 -07:00
compression.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
compression.h btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
ctree.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
ctree.h Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
delayed-inode.c Btrfs: introduce the delayed inode ref deletion for the single link inode 2014-01-28 13:20:09 -08:00
delayed-inode.h Btrfs: introduce the delayed inode ref deletion for the single link inode 2014-01-28 13:20:09 -08:00
delayed-ref.c Btrfs: skip merge part for delayed data refs 2014-01-28 13:19:23 -08:00
delayed-ref.h Btrfs: introduce a head ref rbtree 2014-01-28 13:19:22 -08:00
dev-replace.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
dev-replace.h Btrfs: add new sources for device replace code 2012-12-12 17:15:41 -05:00
dir-item.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
disk-io.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
disk-io.h Btrfs: add a sanity test for btrfs_split_item 2013-11-11 21:51:02 -05:00
export.c btrfs: remove fs/btrfs/compat.h 2013-11-11 22:03:19 -05:00
export.h
extent-tree.c Btrfs: stop caching thread if extent_commit_sem is contended 2014-01-28 13:20:10 -08:00
extent_io.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
extent_io.h Btrfs: move the extent buffer radix tree into the fs_info 2014-01-28 13:19:55 -08:00
extent_map.c Btrfs: fix extent_map block_len after merging 2014-01-28 13:19:51 -08:00
extent_map.h btrfs: Enclose macros with complex values within parenthesis 2013-11-11 22:12:06 -05:00
file-item.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
file.c Btrfs: fix use of uninitialized err variable 2014-01-28 13:19:57 -08:00
free-space-cache.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
free-space-cache.h Btrfs: remove path arg from btrfs_truncate_free_space_cache 2013-11-11 21:51:33 -05:00
hash.h btrfs: extended inode refs 2012-10-09 09:14:45 -04:00
inode-item.c btrfs: cleanup: removed unused 'btrfs_get_inode_ref_index' 2014-01-28 13:19:39 -08:00
inode-map.c btrfs: Use WARN_ON()'s return value in place of WARN_ON(1) 2013-11-11 22:11:53 -05:00
inode-map.h
inode.c Btrfs: introduce the delayed inode ref deletion for the single link inode 2014-01-28 13:20:09 -08:00
ioctl.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
Kconfig Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs 2013-11-22 08:38:55 -08:00
locking.c btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
locking.h Btrfs: remove btrfs_try_spin_lock 2013-03-14 14:57:10 -04:00
lzo.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
Makefile Btrfs: add tests for btrfs_get_extent 2013-11-11 21:57:30 -05:00
math.h Btrfs: cleanup duplicated division functions 2012-12-11 13:31:30 -05:00
ordered-data.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
ordered-data.h Btrfs: don't wait for the completion of all the ordered extents 2013-11-11 22:13:44 -05:00
orphan.c btrfs: expand btrfs_find_item() to include find_orphan_item functionality 2014-01-28 13:19:37 -08:00
print-tree.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
print-tree.h btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
qgroup.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
raid56.c btrfs: remove fs/btrfs/compat.h 2013-11-11 22:03:19 -05:00
raid56.h Btrfs: RAID5 and RAID6 2013-02-01 14:24:23 -05:00
rcu-string.h Btrfs: use rcu to protect device->name 2012-06-14 21:29:16 -04:00
reada.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
relocation.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
root-tree.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
scrub.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
send.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
send.h btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
struct-funcs.c Btrfs: rewrite BTRFS_SETGET_FUNCS 2012-07-23 16:28:06 -04:00
super.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
sysfs.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
sysfs.h btrfs: publish allocation data in sysfs 2014-01-28 13:19:29 -08:00
transaction.c Btrfs: remove btrfs_end_transaction_dmeta() 2014-01-28 13:20:08 -08:00
transaction.h Btrfs: remove btrfs_end_transaction_dmeta() 2014-01-28 13:20:08 -08:00
tree-defrag.c Btrfs: cleanup dead code of defragment 2013-11-11 21:59:45 -05:00
tree-log.c btrfs: expand btrfs_find_item() to include find_orphan_item functionality 2014-01-28 13:19:37 -08:00
tree-log.h btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
ulist.c Btrfs: fix crash regarding to ulist_add_merge 2013-07-02 11:50:59 -04:00
ulist.h Btrfs: add a rb_tree to improve performance of ulist search 2013-05-06 15:54:44 -04:00
uuid-tree.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
volumes.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00
volumes.h btrfs: Pack struct btrfs_device 2013-11-11 22:11:26 -05:00
xattr.c btrfs: make static code static & remove dead code 2013-05-06 15:55:23 -04:00
xattr.h
zlib.c Btrfs: convert printk to btrfs_ and fix BTRFS prefix 2014-01-28 13:20:05 -08:00