Messages in this thread | | | Date | Thu, 17 Feb 2022 21:31:20 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH][RFC] ipc,fs: use rcu_work to free struct ipc_namespace |
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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
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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.
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