* [PATCH] btrfs: Annotate unlocked accesses to transaction state
@ 2020-11-24 14:44 Nikolay Borisov
2021-04-13 21:57 ` Boris Burkov
0 siblings, 1 reply; 2+ messages in thread
From: Nikolay Borisov @ 2020-11-24 14:44 UTC (permalink / raw)
To: linux-btrfs; +Cc: Nikolay Borisov
btrfs_transaction::state is protected by btrfs_fs_info::trans_lock and
all accesses to this variable should be synchornized by it. However,
there are 2 exceptions, namely checking if currently running transaction
has entered its commit phase in start_transaction and in
btrfs_should_end_transaction.
Annotate those 2, unlocked accesses with READ_ONCE respectively. Also
remove the full memory barrier in start_transaction as it provides no
ordering guarantees due to being unpaired. All other accsess to
transaction state happen under trans_lock being held.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
fs/btrfs/transaction.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index df5687a92798..e5a5c3604a9b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -695,8 +695,7 @@ start_transaction(struct btrfs_root *root, unsigned int num_items,
h->can_flush_pending_bgs = true;
INIT_LIST_HEAD(&h->new_bgs);
- smp_mb();
- if (cur_trans->state >= TRANS_STATE_COMMIT_START &&
+ if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START &&
may_wait_transaction(fs_info, type)) {
current->journal_info = h;
btrfs_commit_transaction(h);
@@ -912,7 +911,7 @@ int btrfs_should_end_transaction(struct btrfs_trans_handle *trans)
{
struct btrfs_transaction *cur_trans = trans->transaction;
- if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
+ if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START ||
test_bit(BTRFS_DELAYED_REFS_FLUSHING,
&cur_trans->delayed_refs.flags))
return 1;
--
2.25.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] btrfs: Annotate unlocked accesses to transaction state
2020-11-24 14:44 [PATCH] btrfs: Annotate unlocked accesses to transaction state Nikolay Borisov
@ 2021-04-13 21:57 ` Boris Burkov
0 siblings, 0 replies; 2+ messages in thread
From: Boris Burkov @ 2021-04-13 21:57 UTC (permalink / raw)
To: Nikolay Borisov; +Cc: linux-btrfs
On Tue, Nov 24, 2020 at 04:44:13PM +0200, Nikolay Borisov wrote:
> btrfs_transaction::state is protected by btrfs_fs_info::trans_lock and
> all accesses to this variable should be synchornized by it. However,
> there are 2 exceptions, namely checking if currently running transaction
> has entered its commit phase in start_transaction and in
> btrfs_should_end_transaction.
>
> Annotate those 2, unlocked accesses with READ_ONCE respectively. Also
> remove the full memory barrier in start_transaction as it provides no
> ordering guarantees due to being unpaired. All other accsess to
> transaction state happen under trans_lock being held.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
> fs/btrfs/transaction.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index df5687a92798..e5a5c3604a9b 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -695,8 +695,7 @@ start_transaction(struct btrfs_root *root, unsigned int num_items,
> h->can_flush_pending_bgs = true;
> INIT_LIST_HEAD(&h->new_bgs);
>
> - smp_mb();
> - if (cur_trans->state >= TRANS_STATE_COMMIT_START &&
> + if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START &&
> may_wait_transaction(fs_info, type)) {
> current->journal_info = h;
> btrfs_commit_transaction(h);
> @@ -912,7 +911,7 @@ int btrfs_should_end_transaction(struct btrfs_trans_handle *trans)
> {
> struct btrfs_transaction *cur_trans = trans->transaction;
>
> - if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
> + if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START ||
> test_bit(BTRFS_DELAYED_REFS_FLUSHING,
> &cur_trans->delayed_refs.flags))
> return 1;
> --
> 2.25.1
>
I think some parts of this have already gone in and the conditional has
been lightly refactored, so this patch no longer applies to
kdave/misc-next.
Assuming this is a change for adherence to best practices rather than a
fix for a specific issue (in which case it would be nice to have the
issue described in the message), the patch looks good to me otherwise.
^ permalink raw reply [flat|nested] 2+ messages in thread
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