Commit f7f497cb authored by Ritesh Harjani's avatar Ritesh Harjani Committed by Theodore Ts'o
Browse files

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: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarRitesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/d89e599658b4a1f3893a48c6feded200073037fc.1644992076.git.riteshh@linux.ibm.com


Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent cc16eeca

fs/jbd2/transaction.c

+9 −19
Original line number Diff line number Diff line
@@ -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);

include/linux/jbd2.h

+0 −3
Original line number Diff line number Diff line
@@ -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)

 *

 */