Re: [PATCH 1/2] Updated locking documentation for transaction_t
From: Jan Kara <jack@suse.cz>
Date: 2021-02-11 09:39:57
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On Wed 10-02-21 10:57:39, Alexander Lochmann wrote:
Some members of transaction_t are allowed to be read without any lock being held if consistency doesn't matter. Based on LockDoc's findings, we extended the locking documentation of those members. Each one of them is marked with a short comment: "no lock for quick racy checks". Signed-off-by: Alexander Lochmann <redacted> Signed-off-by: Horst Schirmeier <redacted>
Thanks for the patch! Some comments below...
quoted hunk ↗ jump to hunk
--- include/linux/jbd2.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h index 99d3cd051ac3..18f77d9b1745 100644 --- a/include/linux/jbd2.h +++ b/include/linux/jbd2.h@@ -594,18 +594,18 @@ struct transaction_s */ unsigned long t_log_start; - /* Number of buffers on the t_buffers list [j_list_lock] */ + /* Number of buffers on the t_buffers list [j_list_lock, no lock for quick racy checks] */ int t_nr_buffers;
So this case is actually somewhat different now that I audited the uses. There are two types of users - commit code (fs/jbd2/commit.c) and others. Other users properly use j_list_lock to access t_nr_buffers. Commit code does not use any locks because committing transaction is fully in ownership of the jbd2 thread and all other users need to check & wait for commit to be finished before doing anything with the transaction's buffers.
quoted hunk ↗ jump to hunk
/* * Doubly-linked circular list of all buffers reserved but not yet - * modified by this transaction [j_list_lock] + * modified by this transaction [j_list_lock, no lock for quick racy checks] */ struct journal_head *t_reserved_list; /* * Doubly-linked circular list of all metadata buffers owned by this - * transaction [j_list_lock] + * transaction [j_list_lock, no lock for quick racy checks] */ struct journal_head *t_buffers;@@ -631,7 +631,7 @@ struct transaction_s /* * Doubly-linked circular list of metadata buffers being shadowed by log * IO. The IO buffers on the iobuf list and the shadow buffers on this - * list match each other one for one at all times. [j_list_lock] + * list match each other one for one at all times. [j_list_lock, no lock for quick racy checks] */ struct journal_head *t_shadow_list;
The above three cases are the same as t_reserved_list. Honza -- Jan Kara [off-list ref] SUSE Labs, CR