linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Date:   Sun, 15 Jan 2023 15:46:10 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "parri.andrea" <parri.andrea@...il.com>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Sun, Jan 15, 2023 at 10:10:52AM -0800, Paul E. McKenney wrote:
> On Sun, Jan 15, 2023 at 11:23:31AM -0500, Alan Stern wrote:
> > On Sat, Jan 14, 2023 at 09:15:10PM -0800, Paul E. McKenney wrote:
> > > What am I missing here?
> > 
> > I don't think you're missing anything.  This is a matter for Boqun or 
> > Luc; it must have something to do with the way herd treats the 
> > srcu_read_lock() and srcu_read_unlock() primitives.
> 
> It looks like we need something that tracks (data | rf)* between
> the return value of srcu_read_lock() and the second parameter of
> srcu_read_unlock().  The reason for rf rather than rfi is the upcoming
> srcu_down_read() and srcu_up_read().

Or just make herd treat srcu_read_lock(s) as an annotated equivalent of 
READ_ONCE(&s) and srcu_read_unlock(s, v) as an annotated equivalent of 
WRITE_ONCE(s, v).  But with some special accomodation to avoid 
interaction with the new carry-dep relation.

> But what I will do in the meantime is to switch back to a commit that
> simply flags nesting of same-srcu_struct SRCU read-side critical sections,
> while blindly assuming that the return value of a given srcu_read_lock()
> is passed in to the corresponding srcu_read_unlock():
> 
> ------------------------------------------------------------------------
> 
> (* Compute matching pairs of Srcu-lock and Srcu-unlock, but prohibit nesting *)
> let srcu-unmatched = Srcu-lock | Srcu-unlock
> let srcu-unmatched-po = ([srcu-unmatched] ; po ; [srcu-unmatched]) & loc
> let srcu-unmatched-locks-to-unlock = ([Srcu-lock] ; po ; [Srcu-unlock]) & loc
> let srcu-rscs = srcu-unmatched-locks-to-unlock \ (srcu-unmatched-po ; srcu-unmatched-po)
> 
> (* Validate nesting *)
> flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> 
> (* Check for use of synchronize_srcu() inside an RCU critical section *)
> flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
> 
> (* Validate SRCU dynamic match *)
> flag ~empty different-values(srcu-rscs) as srcu-bad-nesting
> 
> ------------------------------------------------------------------------
> 
> Or is there some better intermediate position that could be taken?

Do you mean go back to the current linux-kernel.bell?  The code you 
wrote above is different, since it prohibits nesting.

Alan