Messages in this thread | | | Date | Mon, 3 Aug 2015 19:30:07 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 4/4] change sb_writers to use percpu_rw_semaphore |
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Hi Jan,
Thanks for your review and sorry for delay, I was on vacation.
On 07/28, Jan Kara wrote: > > On Wed 22-07-15 23:15:41, Oleg Nesterov wrote: > > > > Perhaps we should also cleanup the usage of ->frozen. It would be > > better to set/clear (say) SB_FREEZE_WRITE with the corresponding > > write-lock held. Currently freeze_super() has to set SB_FREEZE_WRITE > > before sb_wait_write(SB_FREEZE_WRITE) to avoid the race with itself, > > we can add another state. The "From now on, no new normal writers > > can start" removed by this patch was not really correct. > > The patch looks good, just one question: Why wasn't the above comment > really correct?
It is not that I think it was wrong, just not 100% accurate even before this change. "w_writers.frozen = SB_FREEZE_WRITE" itself can't guarantee that "no new normal writers can start". We do not know when other CPU's will see the result of this STORE.
> Do you mean it wouldn't be correct after your changes? I > agree with that.
Yes, yes, this was the actual reason to remove this comment. Sorry for confusion.
> Also when you'd like to "cleanup the usage of ->frozen", you have to be > careful no only about races with freeze_super() itself but also about races > with remount (that's one of the reasons why we use s_umount for protecting > modifications of ->frozen). So I'm not sure how much we can actually > improve on code readability...
Yes, me too. Probably I should simply remove this (confusing) part of the changelog.
> Anyway, you can add: > > Reviewed-by: Jan Kara <jack@suse.com>
Thanks!
OK. Now I'll try to actually test this all. Hopefully this week.
Oleg.
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