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Message-Id: <089b38635884e95cfd858c003630bdc8bd7f22b0.1642953021.git.riteshh@linux.ibm.com>
Date: Sun, 23 Jan 2022 22:53:27 +0530
From: Ritesh Harjani <riteshh@...ux.ibm.com>
To: linux-ext4@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
Jan Kara <jack@...e.com>, tytso@....edu,
Ritesh Harjani <riteshh@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: [PATCHv1 1/2] jbd2: Kill t_handle_lock transaction spinlock
This patch kills t_handle_lock transaction spinlock completely from
jbd2.
To explain the reasoning, currently there were three sites at which this
spinlock was used.
1. jbd2_journal_wait_updates()
a. Based on careful code review it can be seen that, we don't need this
lock here. This is since we wait for any currently ongoing updates
based on a atomic variable t_updates. And we anyway don't take any
t_handle_lock while in stop_this_handle().
i.e.
write_lock(&journal->j_state_lock()
jbd2_journal_wait_updates() stop_this_handle()
while (atomic_read(txn->t_updates) { |
DEFINE_WAIT(wait); |
prepare_to_wait(); |
if (atomic_read(txn->t_updates) if (atomic_dec_and_test(txn->t_updates))
write_unlock(&journal->j_state_lock);
schedule(); wake_up()
write_lock(&journal->j_state_lock);
finish_wait();
}
txn->t_state = T_COMMIT
write_unlock(&journal->j_state_lock);
b. Also note that between atomic_inc(&txn->t_updates) in
start_this_handle() and jbd2_journal_wait_updates(), the
synchronization happens via read_lock(journal->j_state_lock) in
start_this_handle();
2. jbd2_journal_extend()
a. jbd2_journal_extend() is called with the handle of each process from
task_struct. So no lock required in updating member fields of handle_t
b. For member fields of h_transaction, all updates happens only via
atomic APIs (which is also within read_lock()).
So, no need of this transaction spinlock.
3. update_t_max_wait()
Based on Jan suggestion, this can be carefully removed using atomic
cmpxchg API.
Note that there can be several processes which are waiting for a new
transaction to be allocated and started. For doing this only one
process will succeed in taking write_lock() and allocating a new txn.
After that all of the process will be updating the t_max_wait (max
transaction wait time). This can be done via below method w/o taking
any locks using atomic cmpxchg.
For more details refer [1]
new = get_new_val();
old = READ_ONCE(ptr->max_val);
while (old < new)
old = cmpxchg(&ptr->max_val, old, new);
[1]: https://lwn.net/Articles/849237/
Suggested-by: Jan Kara <jack@...e.cz>
Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
---
fs/jbd2/transaction.c | 29 +++++++++--------------------
include/linux/jbd2.h | 3 ---
2 files changed, 9 insertions(+), 23 deletions(-)
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 8e2f8275a253..68dd7de49aff 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -107,7 +107,6 @@ static void jbd2_get_transaction(journal_t *journal,
transaction->t_start_time = ktime_get();
transaction->t_tid = journal->j_transaction_sequence++;
transaction->t_expires = jiffies + journal->j_commit_interval;
- spin_lock_init(&transaction->t_handle_lock);
atomic_set(&transaction->t_updates, 0);
atomic_set(&transaction->t_outstanding_credits,
jbd2_descriptor_blocks_per_trans(journal) +
@@ -139,24 +138,21 @@ static void jbd2_get_transaction(journal_t *journal,
/*
* Update transaction's maximum wait time, if debugging is enabled.
*
- * In order for t_max_wait to be reliable, it must be protected by a
- * lock. But doing so will mean that start_this_handle() can not be
- * run in parallel on SMP systems, which limits our scalability. So
- * unless debugging is enabled, we no longer update t_max_wait, which
- * means that maximum wait time reported by the jbd2_run_stats
- * tracepoint will always be zero.
+ * t_max_wait is carefully updated here with use of atomic compare exchange.
+ * Note that there could be multiplre threads trying to do this simultaneously
+ * hence using cmpxchg to avoid any use of locks in this case.
*/
static inline void update_t_max_wait(transaction_t *transaction,
unsigned long ts)
{
#ifdef CONFIG_JBD2_DEBUG
+ unsigned long oldts, newts;
if (jbd2_journal_enable_debug &&
time_after(transaction->t_start, ts)) {
- ts = jbd2_time_diff(ts, transaction->t_start);
- spin_lock(&transaction->t_handle_lock);
- if (ts > transaction->t_max_wait)
- transaction->t_max_wait = ts;
- spin_unlock(&transaction->t_handle_lock);
+ newts = jbd2_time_diff(ts, transaction->t_start);
+ oldts = READ_ONCE(transaction->t_max_wait);
+ while (oldts < newts)
+ oldts = cmpxchg(&transaction->t_max_wait, oldts, newts);
}
#endif
}
@@ -690,7 +686,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records)
DIV_ROUND_UP(
handle->h_revoke_credits_requested,
journal->j_revoke_records_per_block);
- spin_lock(&transaction->t_handle_lock);
wanted = atomic_add_return(nblocks,
&transaction->t_outstanding_credits);
@@ -698,7 +693,7 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records)
jbd_debug(3, "denied handle %p %d blocks: "
"transaction too large\n", handle, nblocks);
atomic_sub(nblocks, &transaction->t_outstanding_credits);
- goto unlock;
+ goto error_out;
}
trace_jbd2_handle_extend(journal->j_fs_dev->bd_dev,
@@ -714,8 +709,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records)
result = 0;
jbd_debug(3, "extended handle %p by %d\n", handle, nblocks);
-unlock:
- spin_unlock(&transaction->t_handle_lock);
error_out:
read_unlock(&journal->j_state_lock);
return result;
@@ -847,22 +840,18 @@ void jbd2_journal_wait_updates(journal_t *journal)
if (!commit_transaction)
return;
- spin_lock(&commit_transaction->t_handle_lock);
while (atomic_read(&commit_transaction->t_updates)) {
DEFINE_WAIT(wait);
prepare_to_wait(&journal->j_wait_updates, &wait,
TASK_UNINTERRUPTIBLE);
if (atomic_read(&commit_transaction->t_updates)) {
- spin_unlock(&commit_transaction->t_handle_lock);
write_unlock(&journal->j_state_lock);
schedule();
write_lock(&journal->j_state_lock);
- spin_lock(&commit_transaction->t_handle_lock);
}
finish_wait(&journal->j_wait_updates, &wait);
}
- spin_unlock(&commit_transaction->t_handle_lock);
}
/**
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 575c3057a98a..500a95f8c914 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -554,9 +554,6 @@ struct transaction_chp_stats_s {
* ->j_list_lock
*
* j_state_lock
- * ->t_handle_lock
- *
- * j_state_lock
* ->j_list_lock (journal_unmap_buffer)
*
*/
--
2.31.1
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