RFC change sb_writers to use percpu_rw_semaphore

Hello,

Al, Jan, could you comment? I mean the intent, the patches are obviously not for inclusion yet.

We can remove everything from struct sb_writers except frozen (which can become a boolean, it seems) and add the array of percpu_rw_semaphore's instead.

__sb_start/end_write() can use percpu_down/up_read(), and freeze/thaw_super() can use percpu_down/up_write().

Why:

- Firstly, __sb_start_write() looks simply buggy. I does __sb_end_write() if it sees ->frozen, but if it migrates to another CPU before percpu_counter_dec() sb_wait_write() can wrongly succeed if there is another task which holds the same "semaphore": sb_wait_write() can miss the result of the previous percpu_counter_inc() but see the result of this percpu_counter_dec().

- This code doesn't look simple. It would be better to rely on the generic locking code.

- __sb_start_write() will be a little bit faster, but this is minor.

Todo:

- __sb_start_write(wait => false) always fail.

Thivial, we already have percpu_down_read_trylock() just this patch wasn't merged yet.

- sb_lockdep_release() and sb_lockdep_acquire() play with percpu_rw_semaphore's internals.

Trivial, we need a couple of new helper in percpu-rwsem.c.

- Fix get_super_thawed(), it will spin if MS_RDONLY...

It is not clear to me what exactly should we do, but this doesn't look hard. Perhaps it can just return if MS_RDONLY.

- Most probably I missed something else, and I do not need how to test.

Finally. freeze_super() calls synchronize_sched_expedited() 3 times in a row. This is bad and just stupid. But if we change percpu_rw_semaphore to use rcu_sync (see https://lkml.org/lkml/2015/7/11/211) we can avoid this and do synchronize_sched() only once. Just we need some more simple changes in percpu-rwsem.c, so that all sb_writers->rw_sem[] semaphores could use the single sb_writers->rss.

In this case destroy_super() needs some modifications too, percpu_free_rwsem() will be might_sleep(). But this looks simple too.

Oleg.

 fs/super.c         |  147 +++++++++++++++++++--------------------------------
 include/linux/fs.h |   14 +----
 2 files changed, 58 insertions(+), 103 deletions(-)