5.15-stable review patch. If anyone has any objections, please let me know.
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From: Ritesh Harjani riteshh@linux.ibm.com
[ Upstream commit f7f497cb702462e8505ff3d8d4e7722ad95626a1 ]
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@suse.cz Signed-off-by: Ritesh Harjani riteshh@linux.ibm.com Reviewed-by: Jan Kara jack@suse.cz Link: https://lore.kernel.org/r/d89e599658b4a1f3893a48c6feded200073037fc.164499207... Signed-off-by: Theodore Ts'o tytso@mit.edu Stable-dep-of: 2dfba3bb40ad ("jbd2: correct the end of the journal recovery scan range") Signed-off-by: Sasha Levin sashal@kernel.org --- fs/jbd2/transaction.c | 28 +++++++++------------------- include/linux/jbd2.h | 3 --- 2 files changed, 9 insertions(+), 22 deletions(-)
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c index 59292b82d4424..b31145b2bb6bf 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; @@ -860,15 +853,12 @@ void jbd2_journal_wait_updates(journal_t *journal) if (!transaction) break;
- spin_lock(&transaction->t_handle_lock); prepare_to_wait(&journal->j_wait_updates, &wait, TASK_UNINTERRUPTIBLE); if (!atomic_read(&transaction->t_updates)) { - spin_unlock(&transaction->t_handle_lock); finish_wait(&journal->j_wait_updates, &wait); break; } - spin_unlock(&transaction->t_handle_lock); write_unlock(&journal->j_state_lock); schedule(); finish_wait(&journal->j_wait_updates, &wait); diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h index 03d8ba98cbeb7..f29fda630b2de 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) * */