On 2021/03/12 0:54, Marco Elver wrote:

quoted

But the more we could have the compiler automatically figure out
things without needing an explicit tag, it would seem to me that this
would be better, since manual tagging is going to be more error-prone.

What you're alluding to here would go much further than a data race
detector ("data race" is still just defined by the memory model). The
wish that there was a static analysis tool that would automatically
understand the "concurrency semantics as intended by the developer" is
something that'd be nice to have, but just doesn't seem realistic.
Because how can a tool tell what the developer intended, without input
from that developer?

Input from developers is very important for not only compilers and tools
but also allowing bug-explorers to understand what is happening.
ext4 currently has

  possible deadlock in start_this_handle (2)
  https://syzkaller.appspot.com/bug?id=38c060d5757cbc13fdffd46e80557c645fbe79ba

which even maintainers cannot understand what is happening.
How can bug-explorers know implicit logic which maintainers believe safe and correct?
It is possible that some oversight in implicit logic is the cause of
"possible deadlock in start_this_handle (2)".
Making implicit assumptions clear helps understanding.

Will "KCSAN: data-race in start_this_handle / start_this_handle" be addressed by marking?
syzbot is already waiting for
"KCSAN: data-race in jbd2_journal_dirty_metadata / jbd2_journal_dirty_metadata" at
https://syzkaller.appspot.com/bug?id=5eb10023f53097f003e72c6a7c1a6f14b7c22929 .

If there's worry marking accesses is error-prone, then that might be a
signal that the concurrency design is too complex (or the developer
hasn't considered all cases).

For that reason, we need to mark accesses to tell the compiler and
tooling where to expect concurrency, so that 1) the compiler generates
correct code, and 2) tooling such as KCSAN can double-check what the
developer intended is actually what's happening.

and 3) bug-explorers can understand what the developers are assuming/missing.