On Fri, Feb 18, 2022 at 03:29:56AM +0000, Al Viro wrote:
> On Thu, Feb 17, 2022 at 03:36:20PM -0500, Rik van Riel wrote:
> > The patch works, but a cleanup question for Al Viro:
> >
> > How do we get rid of #include "../fs/mount.h" and the raw ->mnt_ns = NULL thing
> > in the cleanest way?
>
> Hell knows... mnt_make_shortterm(mnt) with big, fat warning along the lines of
> "YOU MUST HAVE AN RCU GRACE PERIOD BEFORE YOU DROP THAT REFERENCE!!!", perhaps?
Rik's patch uses queue_rcu_work(), which always uses a normal grace
period. Therefore, one way of checking that an RCU grace period has
elapsed is as follows:
Step 1: Get a snapshot of the normal grace-period state:
rcuseq = get_state_synchronize_rcu(); // In mainline
Step 2: Verify that a normal grace period has elapsed since step 1:
WARN_ON_ONCE(!poll_state_synchronize_rcu(rcuseq));
These functions are both in mainline.
And apologies for my answer on IRC being unhelpful. Here is hoping that
this is more to the point.
Thanx, Paul
------------------------------------------------------------------------
PS. Just in case it ever becomes relevant, if Rik's patch were instead
to use synchronize_rcu() or synchronize_rcu_expedited() to wait for
the grace period, it would be necessary to capture both the normal
and expedited grace-period state:
Step 1: Get a snapshot of both the normal and the expedited state:
rcuseq = get_state_synchronize_rcu();
rcuxseq = get_state_synchronize_rcu_expedited();
Step 2: Verify that either a normal or expedited grace period has
elapsed since step 1:
WARN_ON_ONCE(!poll_state_synchronize_rcu(rcuseq) &&
!poll_state_synchronize_rcu_expedited(rcuxseq));
The reason for doing both is that synchronize_rcu_expedited() and
synchronize_rcu() can both be switched between using normal and expedited
grace periods. Not just at boot time, but also at runtime. Fun.
The two expedited functions are in -rcu rather than mainline, so if
someone does ever need them, please let me know.
