Silencing false lockdep warning related to seq lock

[Joel Fernandes <joelaf@google.com>]

Hi Boqun,
You might have worked on such issues so I thought you're a good person to ask.

After apply Laurent's SPF patchset [1] , we're facing a large number
of (seemingly false positive) lockdep reports which are related to
circular dependencies with seq locks.

 lock(A); write_seqcount(B)
  vs.
write_seqcount(B); lock(A)

This cannot deadlock obviously. My current strategy which I hate is to
make it a raw seqcount write which bypasses lockdep. That's horrible
for obvious reasons. Do you have any tricks/patches up your sleeve to
silence these?

I suppose we still want to catch lockdep issues of the form (which
peterz chatted to me about):

 lock(A); write_seqcount(B)
  vs.
read_seqcount(B); lock(A)

which seems like it can deadlock.

I would rather make lockdep useful to catch these and not miss out on
them. Let me know what you think?

Cheers,
-Joel

[1] https://lkml.org/lkml/2019/4/16/615