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(-)
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