* [PATCH v2 2/3] btrfs: add barriers to btrfs_sync_log before log_commit_wait wakeups
2018-04-24 13:10 [PATCH v2 0/3] Cleanup waitqueue_active and barriers David Sterba
2018-04-24 13:10 ` [PATCH v2 1/3] btrfs: introduce conditional wakeup helpers David Sterba
@ 2018-04-24 13:11 ` David Sterba
2018-04-24 13:11 ` [PATCH v2 3/3] btrfs: replace waitqueue_actvie with cond_wake_up David Sterba
2 siblings, 0 replies; 4+ messages in thread
From: David Sterba @ 2018-04-24 13:11 UTC (permalink / raw)
To: linux-btrfs; +Cc: David Sterba, Nikolay Borisov
Currently the code assumes that there's an implied barrier by the
sequence of code preceding the wakeup, namely the mutex unlock.
As Nikolay pointed out:
I think this is wrong (not your code) but the original assumption that
the RELEASE semantics provided by mutex_unlock is sufficient.
According to memory-barriers.txt:
Section 'LOCK ACQUISITION FUNCTIONS' states:
(2) RELEASE operation implication:
Memory operations issued before the RELEASE will be completed before the
RELEASE operation has completed.
Memory operations issued after the RELEASE *may* be completed before the
RELEASE operation has completed.
(I've bolded the may portion)
The example given there:
As an example, consider the following:
*A = a;
*B = b;
ACQUIRE
*C = c;
*D = d;
RELEASE
*E = e;
*F = f;
The following sequence of events is acceptable:
ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
So if we assume that *C is modifying the flag which the waitqueue is checking,
and *E is the actual wakeup, then those accesses can be re-ordered...
IMHO this code should be considered broken...
~~~
To be on the safe side, add the barriers. The synchronization logic
around log using the mutexes and several other threads does not make it
easy to reason for/against the barrier.
CC: Nikolay Borisov <nborisov@suse.com>
Link: https://lkml.kernel.org/r/6ee068d8-1a69-3728-00d1-d86293d43c9f@suse.com
Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/tree-log.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 43758e30aa7a..fa5b3dc5f4d5 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3116,8 +3116,11 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
mutex_unlock(&log_root_tree->log_mutex);
/*
- * The barrier before waitqueue_active is implied by mutex_unlock
+ * The barrier before waitqueue_active is needed so all the updates
+ * above are seen by the woken threads. It might not be necessary, but
+ * proving that seems to be hard.
*/
+ smp_mb();
if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
wake_up(&log_root_tree->log_commit_wait[index2]);
out:
@@ -3128,8 +3131,11 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
mutex_unlock(&root->log_mutex);
/*
- * The barrier before waitqueue_active is implied by mutex_unlock
+ * The barrier before waitqueue_active is needed so all the updates
+ * above are seen by the woken threads. It might not be necessary, but
+ * proving that seems to be hard.
*/
+ smp_mb();
if (waitqueue_active(&root->log_commit_wait[index1]))
wake_up(&root->log_commit_wait[index1]);
return ret;
--
2.16.2
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v2 3/3] btrfs: replace waitqueue_actvie with cond_wake_up
2018-04-24 13:10 [PATCH v2 0/3] Cleanup waitqueue_active and barriers David Sterba
2018-04-24 13:10 ` [PATCH v2 1/3] btrfs: introduce conditional wakeup helpers David Sterba
2018-04-24 13:11 ` [PATCH v2 2/3] btrfs: add barriers to btrfs_sync_log before log_commit_wait wakeups David Sterba
@ 2018-04-24 13:11 ` David Sterba
2 siblings, 0 replies; 4+ messages in thread
From: David Sterba @ 2018-04-24 13:11 UTC (permalink / raw)
To: linux-btrfs; +Cc: David Sterba
Use the wrappers and reduce the amount of low-level details about the
waitqueue management.
Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/compression.c | 7 +------
fs/btrfs/delayed-inode.c | 9 +++------
fs/btrfs/dev-replace.c | 10 ++++------
fs/btrfs/extent-tree.c | 7 +------
fs/btrfs/inode.c | 9 +++------
fs/btrfs/locking.c | 34 +++++++++++-----------------------
fs/btrfs/ordered-data.c | 14 ++++----------
fs/btrfs/transaction.c | 7 +------
fs/btrfs/tree-log.c | 23 +++++++----------------
9 files changed, 35 insertions(+), 85 deletions(-)
diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 5268c9f85ca7..dcebb91e4e0c 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -990,12 +990,7 @@ static void __free_workspace(int type, struct list_head *workspace,
btrfs_compress_op[idx]->free_workspace(workspace);
atomic_dec(total_ws);
wake:
- /*
- * Make sure counter is updated before we wake up waiters.
- */
- smp_mb();
- if (waitqueue_active(ws_wait))
- wake_up(ws_wait);
+ cond_wake_up(ws_wait);
}
static void free_workspace(int type, struct list_head *ws)
diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
index a8d492dbd3e7..3b987cb94c4f 100644
--- a/fs/btrfs/delayed-inode.c
+++ b/fs/btrfs/delayed-inode.c
@@ -460,13 +460,10 @@ static void finish_one_item(struct btrfs_delayed_root *delayed_root)
{
int seq = atomic_inc_return(&delayed_root->items_seq);
- /*
- * atomic_dec_return implies a barrier for waitqueue_active
- */
+ /* atomic_dec_return implies a barrier for waitqueue_active */
if ((atomic_dec_return(&delayed_root->items) <
- BTRFS_DELAYED_BACKGROUND || seq % BTRFS_DELAYED_BATCH == 0) &&
- waitqueue_active(&delayed_root->wait))
- wake_up(&delayed_root->wait);
+ BTRFS_DELAYED_BACKGROUND || seq % BTRFS_DELAYED_BATCH == 0))
+ cond_wake_up_nomb(&delayed_root->wait);
}
static void __btrfs_remove_delayed_item(struct btrfs_delayed_item *delayed_item)
diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index f82be266ba4b..d33ff5b23302 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -916,9 +916,9 @@ void btrfs_dev_replace_clear_lock_blocking(
ASSERT(atomic_read(&dev_replace->read_locks) > 0);
ASSERT(atomic_read(&dev_replace->blocking_readers) > 0);
read_lock(&dev_replace->lock);
- if (atomic_dec_and_test(&dev_replace->blocking_readers) &&
- waitqueue_active(&dev_replace->read_lock_wq))
- wake_up(&dev_replace->read_lock_wq);
+ /* Barrier implied by atomic_dec_and_test */
+ if (atomic_dec_and_test(&dev_replace->blocking_readers))
+ cond_wake_up_nomb(&dev_replace->read_lock_wq);
}
void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info *fs_info)
@@ -929,9 +929,7 @@ void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info *fs_info)
void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount)
{
percpu_counter_sub(&fs_info->bio_counter, amount);
-
- if (waitqueue_active(&fs_info->replace_wait))
- wake_up(&fs_info->replace_wait);
+ cond_wake_up_nomb(&fs_info->replace_wait);
}
void btrfs_bio_counter_inc_blocked(struct btrfs_fs_info *fs_info)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index f99102063366..c4755e8c13a5 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -11087,12 +11087,7 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
{
percpu_counter_dec(&root->subv_writers->counter);
- /*
- * Make sure counter is updated before we wake up waiters.
- */
- smp_mb();
- if (waitqueue_active(&root->subv_writers->wait))
- wake_up(&root->subv_writers->wait);
+ cond_wake_up(&root->subv_writers->wait);
}
int btrfs_start_write_no_snapshotting(struct btrfs_root *root)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index d241285a0d2a..cf6f2815ffee 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1156,13 +1156,10 @@ static noinline void async_cow_submit(struct btrfs_work *work)
nr_pages = (async_cow->end - async_cow->start + PAGE_SIZE) >>
PAGE_SHIFT;
- /*
- * atomic_sub_return implies a barrier for waitqueue_active
- */
+ /* atomic_sub_return implies a barrier */
if (atomic_sub_return(nr_pages, &fs_info->async_delalloc_pages) <
- 5 * SZ_1M &&
- waitqueue_active(&fs_info->async_submit_wait))
- wake_up(&fs_info->async_submit_wait);
+ 5 * SZ_1M)
+ cond_wake_up_nomb(&fs_info->async_submit_wait);
if (async_cow->inode)
submit_compressed_extents(async_cow->inode, async_cow);
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index e4faefac9d16..1da768e5ef75 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -66,22 +66,16 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
write_lock(&eb->lock);
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_inc(&eb->spinning_writers);
- /*
- * atomic_dec_and_test implies a barrier for waitqueue_active
- */
- if (atomic_dec_and_test(&eb->blocking_writers) &&
- waitqueue_active(&eb->write_lock_wq))
- wake_up(&eb->write_lock_wq);
+ /* atomic_dec_and_test implies a barrier */
+ if (atomic_dec_and_test(&eb->blocking_writers))
+ cond_wake_up_nomb(&eb->write_lock_wq);
} else if (rw == BTRFS_READ_LOCK_BLOCKING) {
BUG_ON(atomic_read(&eb->blocking_readers) == 0);
read_lock(&eb->lock);
atomic_inc(&eb->spinning_readers);
- /*
- * atomic_dec_and_test implies a barrier for waitqueue_active
- */
- if (atomic_dec_and_test(&eb->blocking_readers) &&
- waitqueue_active(&eb->read_lock_wq))
- wake_up(&eb->read_lock_wq);
+ /* atomic_dec_and_test implies a barrier */
+ if (atomic_dec_and_test(&eb->blocking_readers))
+ cond_wake_up_nomb(&eb->read_lock_wq);
}
}
@@ -221,12 +215,9 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
}
btrfs_assert_tree_read_locked(eb);
WARN_ON(atomic_read(&eb->blocking_readers) == 0);
- /*
- * atomic_dec_and_test implies a barrier for waitqueue_active
- */
- if (atomic_dec_and_test(&eb->blocking_readers) &&
- waitqueue_active(&eb->read_lock_wq))
- wake_up(&eb->read_lock_wq);
+ /* atomic_dec_and_test implies a barrier */
+ if (atomic_dec_and_test(&eb->blocking_readers))
+ cond_wake_up_nomb(&eb->read_lock_wq);
atomic_dec(&eb->read_locks);
}
@@ -275,12 +266,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
if (blockers) {
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_dec(&eb->blocking_writers);
- /*
- * Make sure counter is updated before we wake up waiters.
- */
+ /* Use the lighter barrier after atomic */
smp_mb__after_atomic();
- if (waitqueue_active(&eb->write_lock_wq))
- wake_up(&eb->write_lock_wq);
+ cond_wake_up_nomb(&eb->write_lock_wq);
} else {
WARN_ON(atomic_read(&eb->spinning_writers) != 1);
atomic_dec(&eb->spinning_writers);
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 6db8bb2f2c28..2e1a1694a33d 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -343,11 +343,8 @@ int btrfs_dec_test_first_ordered_pending(struct inode *inode,
if (entry->bytes_left == 0) {
ret = test_and_set_bit(BTRFS_ORDERED_IO_DONE, &entry->flags);
- /*
- * Implicit memory barrier after test_and_set_bit
- */
- if (waitqueue_active(&entry->wait))
- wake_up(&entry->wait);
+ /* test_and_set_bit implies a barrier */
+ cond_wake_up_nomb(&entry->wait);
} else {
ret = 1;
}
@@ -410,11 +407,8 @@ int btrfs_dec_test_ordered_pending(struct inode *inode,
if (entry->bytes_left == 0) {
ret = test_and_set_bit(BTRFS_ORDERED_IO_DONE, &entry->flags);
- /*
- * Implicit memory barrier after test_and_set_bit
- */
- if (waitqueue_active(&entry->wait))
- wake_up(&entry->wait);
+ /* test_and_set_bit implies a barrier */
+ cond_wake_up_nomb(&entry->wait);
} else {
ret = 1;
}
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c944b4769e3c..ff841abb756e 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -877,12 +877,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
atomic_dec(&cur_trans->num_writers);
extwriter_counter_dec(cur_trans, trans->type);
- /*
- * Make sure counter is updated before we wake up waiters.
- */
- smp_mb();
- if (waitqueue_active(&cur_trans->writer_wait))
- wake_up(&cur_trans->writer_wait);
+ cond_wake_up(&cur_trans->writer_wait);
btrfs_put_transaction(cur_trans);
if (current->journal_info == trans)
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index fa5b3dc5f4d5..49c897daaedb 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -222,11 +222,8 @@ int btrfs_pin_log_trans(struct btrfs_root *root)
void btrfs_end_log_trans(struct btrfs_root *root)
{
if (atomic_dec_and_test(&root->log_writers)) {
- /*
- * Implicit memory barrier after atomic_dec_and_test
- */
- if (waitqueue_active(&root->log_writer_wait))
- wake_up(&root->log_writer_wait);
+ /* atomic_dec_and_test implies a barrier */
+ cond_wake_up_nomb(&root->log_writer_wait);
}
}
@@ -2988,11 +2985,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
mutex_lock(&log_root_tree->log_mutex);
if (atomic_dec_and_test(&log_root_tree->log_writers)) {
- /*
- * Implicit memory barrier after atomic_dec_and_test
- */
- if (waitqueue_active(&log_root_tree->log_writer_wait))
- wake_up(&log_root_tree->log_writer_wait);
+ /* atomic_dec_and_test implies a barrier */
+ cond_wake_up_nomb(&log_root_tree->log_writer_wait);
}
if (ret) {
@@ -3120,9 +3114,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
* above are seen by the woken threads. It might not be necessary, but
* proving that seems to be hard.
*/
- smp_mb();
- if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
- wake_up(&log_root_tree->log_commit_wait[index2]);
+ cond_wake_up(&log_root_tree->log_commit_wait[index2]);
out:
mutex_lock(&root->log_mutex);
btrfs_remove_all_log_ctxs(root, index1, ret);
@@ -3135,9 +3127,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
* above are seen by the woken threads. It might not be necessary, but
* proving that seems to be hard.
*/
- smp_mb();
- if (waitqueue_active(&root->log_commit_wait[index1]))
- wake_up(&root->log_commit_wait[index1]);
+ /* The barrier is implied by mutex_unlock */
+ cond_wake_up_nomb(&root->log_commit_wait[index1]);
return ret;
}
--
2.16.2
^ permalink raw reply related [flat|nested] 4+ messages in thread