On 8.03.2018 13:49, David Sterba wrote:
> Use the wrappers and reduce the amount of low-level details about the
> waitqueue management.
>
> Signed-off-by: David Sterba <dste...@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 | 28 ++++++++--------------------
> 9 files changed, 36 insertions(+), 89 deletions(-)
>
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index 562c3e633403..2d2d7380d381 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -1003,12 +1003,7 @@ 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 d06bef16ebd5..3e7f5f26ff0f 100644
> --- a/fs/btrfs/delayed-inode.c
> +++ b/fs/btrfs/delayed-inode.c
> @@ -472,13 +472,10 @@ 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 e279f04b3388..f498572155f1 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -928,9 +928,9 @@ 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)
> @@ -941,9 +941,7 @@ 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);
nit/offtopic:
I think here the code requires comments since we have 2 types of waiters for
fs_info->replace_wait. One is dependent on the percpu_counter_sum (i.e. the
btrfs_rm_dev_replace_blocked). And then there is another condition on the same
wait entry - the btrfs_bio_counter_inc_blocked i.e:
wait_event(fs_info->replace_wait,
!test_bit(BTRFS_FS_STATE_DEV_REPLACING,
&fs_info->fs_state));
geez, who would come up with such synchronization ...
> }
>
> 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 2760292e1175..d57801711884 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -10999,12 +10999,7 @@ 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 fc5b7d82b842..b963b5b4734e 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -1168,13 +1168,10 @@ 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 621083f8932c..cce666cd104e 100644
> --- a/fs/btrfs/locking.c
> +++ b/fs/btrfs/locking.c
> @@ -78,22 +78,16 @@ 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);
> }
> }
>
> @@ -233,12 +227,9 @@ 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);
> }
>
> @@ -287,12 +278,9 @@ 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 9be98e42cfb6..66ff1419e2e0 100644
> --- a/fs/btrfs/ordered-data.c
> +++ b/fs/btrfs/ordered-data.c
> @@ -356,11 +356,8 @@ 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;
> }
> @@ -423,11 +420,8 @@ 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 eab15777ba88..f431223196a0 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -909,12 +909,7 @@ 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 7b8fee45b29e..979fc02214d4 100644
> --- a/fs/btrfs/tree-log.c
> +++ b/fs/btrfs/tree-log.c
> @@ -235,11 +235,8 @@ 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);
> }
> }
>
> @@ -2965,11 +2962,8 @@ 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) {
> @@ -3092,11 +3086,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
> atomic_set(&log_root_tree->log_commit[index2], 0);
> mutex_unlock(&log_root_tree->log_mutex);
>
> - /*
> - * The barrier before waitqueue_active is implied by mutex_unlock
> - */
> - if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
> - wake_up(&log_root_tree->log_commit_wait[index2]);
> + /* The barrier is implied by mutex_unlock */
> + cond_wake_up_nomb(&log_root_tree->log_commit_wait[index2]);
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...
> out:
> mutex_lock(&root->log_mutex);
> btrfs_remove_all_log_ctxs(root, index1, ret);
> @@ -3104,11 +3095,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
> atomic_set(&root->log_commit[index1], 0);
> mutex_unlock(&root->log_mutex);
>
> - /*
> - * The barrier before waitqueue_active is implied by mutex_unlock
> - */
> - 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]);
ditto.
> return ret;
> }
>
>
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