Re: [RFC] Fine-grained locking documentation for jbd2 data structures

On Tue 09-02-21 10:58:48, Alexander Lochmann wrote:

On 08.02.21 16:27, Jan Kara wrote:

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Hi Alexander!

On Fri 05-02-21 16:31:54, Alexander Lochmann wrote:

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have you had the chance to review our results?

It fell through the cracks I guess. Thanks for pinging. Let me have a look.

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On 15.10.20 15:56, Alexander Lochmann wrote:

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Hi folks,

when comparing our generated locking documentation with the current
documentation
located in include/linux/jbd2.h, I found some inconsistencies. (Our
approach: https://dl.acm.org/doi/10.1145/3302424.3303948)
According to the official documentation, the following members should be
read using a lock:
journal_t
- j_flags: j_state_lock
- j_barrier_count: j_state_lock
- j_running_transaction: j_state_lock
- j_commit_sequence: j_state_lock
- j_commit_request: j_state_lock
transactiont_t
- t_nr_buffers: j_list_lock
- t_buffers: j_list_lock
- t_reserved_list: j_list_lock
- t_shadow_list: j_list_lock
jbd2_inode
- i_transaction: j_list_lock
- i_next_transaction: j_list_lock
- i_flags: j_list_lock
- i_dirty_start: j_list_lock
- i_dirty_start: j_list_lock

However, our results say that no locks are needed at all for *reading*
those members.
 From what I know, it is common wisdom that word-sized data types can be
read without any lock in the Linux kernel.

Yes, although in last year, people try to convert these unlocked reads to
READ_ONCE() or similar as otherwise the compiler is apparently allowed to
generate code which is not safe. But that's a different story.

Is this ongoing work?

Yes, in a way. It's mostly prompted by KCSAN warnings generated by syzbot
;).

Using such a macro would a) make our work much easier as we can instrument
them, and b) would tell less experienced developers that no locking is
needed.

Yes, I agree that it has some benefit for documentation and automatic
checkers as well. OTOH code readability is sometimes hurt by this...

Does the usage of READ_ONCE() imply that no lock is needed?

No, but it does indicate there's something unusual happening with the
variable - usually that variable write can race with this read. 

Otherwise, one could introduce another macro for jbd2, such as #define
READ_UNLOCKED() READ_ONCE(), which is more precise.

Well, yes, but OTOH special macros for small subsystems like this are making
more harm than good in terms of readability since people have to lookup
what exactly they mean anyway.

 Also note

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that although reading that particular word may be safe without any other
locks, the lock still may be needed to safely interpret the value in the
context of other fetched values (e.g., due to consistency among multiple
structure members).

Just a side quest: Do you have an example for such a situation?

Definitely. The simplest case is: You can fetch
journal->j_running_transaction pointer any time without any problem. But
you can *dereference* it only if you hold the j_state_lock while fetching the
pointer and dereferencing it.

So sometimes requiring the lock is just the least

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problematic solution - there's always the tradeoff between the speed and
simplicity.

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All of the above members have word size, i.e., int, long, or ptr.
Is it therefore safe to split the locking documentation as follows?
@j_flags: General journaling state flags [r:nolocks, w:j_state_lock]

I've checked the code and we usually use unlocked reads for quick, possibly
racy checks and if they indicate we may need to do something then take the
lock and do a reliable check. This is quite common pattern, not sure how to
best document this. Maybe like [j_state_lock, no lock for quick racy checks]?

Yeah, I'm fine with that. Does this rule apply for the other members of
journal_t (and transaction_t?) listed above?

Yes.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR