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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.1644992076.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
f7f497cb70