On 09.02.21 13:00, Jan Kara wrote:

quoted

quoted

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
;).

quoted

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...

quoted

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.

quoted

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.

So the only option left would be a global macro such as READ_ONCE() I guess.
How hard would it be to establish such a global notation?
It would make things a lot easier for LockDoc, because we can instrument 
such a macro, and therefore can annotate those accesses.>

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.

Thx.

quoted

So sometimes requiring the lock is just the least

quoted

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

quoted

quoted

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.

Thx. I'll submit a patch for those elements.
For now, this will improve LockDoc's results as we can add "no locks 
needed" to our config for j_flags. We check whether the observed 
accesses match the documented locking rules.
LockDoc will accept both results "j_list_lock" and "no locks needed" for 
reading j_flags.
However, real faulty unlocked accesses will be concealed. :-(

- Alex

								Honza

-- 
Technische Universität Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116
http://ess.cs.tu-dortmund.de/Staff/al