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

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Message-ID: <20230120173054.GM2948950@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 20 Jan 2023 09:30:54 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, 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 Fri, Jan 20, 2023 at 11:14:06AM -0500, Alan Stern wrote:
> On Thu, Jan 19, 2023 at 07:55:21PM -0800, Paul E. McKenney wrote:
> > And for some initial tests:
> > 
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-1.litmus
> > 
> > 	"Flag multiple-srcu-matches" but otherwise OK.
> > 	As a "hail Mary" exercise, I used r4 for the second SRCU
> > 	read-side critical section, but this had no effect.
> > 	(This flag is expected and seen for #4 below.)
> 
> Jonas is right about the reason for this.  Also, his suggestion for 
> fixing the check in lock.cat makes sense.

Very good!

> My revised patch is below.

Thank you!  Are you OK with my putting this on a not-for-mainline branch
for experimental purposes?

> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-2.litmus
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-3.litmus
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-4.litmus
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-5.litmus
> > 
> > 	All as expected.
> > 
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-6.litmus
> > 
> > 	Get "Flag unbalanced-srcu-lock" and "Flag unbalanced-srcu-unlock",
> > 	but this is srcu_down_read() and srcu_up_read(), where this should
> > 	be OK.	Ah, but I need to do the release/acquire/filter trick.  Once
> > 	I did that, it works as expected.
> > 
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-7.litmus
> > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-8.litmus
> > 
> > 	Both as expected.
> > 
> > Getting there!!!
> 
> Good news.

And all of the litmus-repo tests up to ten processes passed.  Woo-hoo!!!

							Thanx, Paul

> Alan
> 
> 
> 
> Index: usb-devel/tools/memory-model/linux-kernel.bell
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.bell
> +++ usb-devel/tools/memory-model/linux-kernel.bell
> @@ -53,38 +53,31 @@ let rcu-rscs = let rec
>  	in matched
>  
>  (* Validate nesting *)
> -flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-locking
> -flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-locking
> +flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-lock
> +flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-unlock
>  
>  (* Compute matching pairs of nested Srcu-lock and Srcu-unlock *)
> -let srcu-rscs = let rec
> -	    unmatched-locks = Srcu-lock \ domain(matched)
> -	and unmatched-unlocks = Srcu-unlock \ range(matched)
> -	and unmatched = unmatched-locks | unmatched-unlocks
> -	and unmatched-po = ([unmatched] ; po ; [unmatched]) & loc
> -	and unmatched-locks-to-unlocks =
> -		([unmatched-locks] ; po ; [unmatched-unlocks]) & loc
> -	and matched = matched | (unmatched-locks-to-unlocks \
> -		(unmatched-po ; unmatched-po))
> -	in matched
> +let carry-srcu-data = (data ; [~ Srcu-unlock] ; rf)*
> +let srcu-rscs = ([Srcu-lock] ; carry-srcu-data ; data ; [Srcu-unlock]) & loc
>  
>  (* Validate nesting *)
> -flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> -flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> +flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-lock
> +flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-unlock
> +flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-srcu-matches
>  
>  (* 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
> +flag ~empty different-values(srcu-rscs) as bad-srcu-value-match
>  
>  (* Compute marked and plain memory accesses *)
>  let Marked = (~M) | IW | Once | Release | Acquire | domain(rmw) | range(rmw) |
> -		LKR | LKW | UL | LF | RL | RU
> + 		LKR | LKW | UL | LF | RL | RU | Srcu-lock | Srcu-unlock
>  let Plain = M \ Marked
>  
>  (* Redefine dependencies to include those carried through plain accesses *)
> -let carry-dep = (data ; rfi)*
> +let carry-dep = (data ; [~ Srcu-unlock] ; rfi)*
>  let addr = carry-dep ; addr
>  let ctrl = carry-dep ; ctrl
>  let data = carry-dep ; data
> Index: usb-devel/tools/memory-model/linux-kernel.def
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.def
> +++ usb-devel/tools/memory-model/linux-kernel.def
> @@ -49,8 +49,10 @@ synchronize_rcu() { __fence{sync-rcu}; }
>  synchronize_rcu_expedited() { __fence{sync-rcu}; }
>  
>  // SRCU
> -srcu_read_lock(X)  __srcu{srcu-lock}(X)
> -srcu_read_unlock(X,Y) { __srcu{srcu-unlock}(X,Y); }
> +srcu_read_lock(X) __load{srcu-lock}(*X)
> +srcu_read_unlock(X,Y) { __store{srcu-unlock}(*X,Y); }
> +srcu_down_read(X) __load{srcu-lock}(*X)
> +srcu_up_read(X,Y) { __store{srcu-unlock}(*X,Y); }
>  synchronize_srcu(X)  { __srcu{sync-srcu}(X); }
>  synchronize_srcu_expedited(X)  { __srcu{sync-srcu}(X); }
>  
> Index: usb-devel/tools/memory-model/lock.cat
> ===================================================================
> --- usb-devel.orig/tools/memory-model/lock.cat
> +++ usb-devel/tools/memory-model/lock.cat
> @@ -36,9 +36,9 @@ let RU = try RU with emptyset
>  (* Treat RL as a kind of LF: a read with no ordering properties *)
>  let LF = LF | RL
>  
> -(* There should be no ordinary R or W accesses to spinlocks *)
> -let ALL-LOCKS = LKR | LKW | UL | LF | RU
> -flag ~empty [M \ IW] ; loc ; [ALL-LOCKS] as mixed-lock-accesses
> +(* There should be no ordinary R or W accesses to spinlocks or SRCU structs *)
> +let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock | Sync-srcu
> +flag ~empty [M \ IW \ ALL-LOCKS] ; loc ; [ALL-LOCKS] as mixed-lock-accesses
>  
>  (* Link Lock-Reads to their RMW-partner Lock-Writes *)
>  let lk-rmw = ([LKR] ; po-loc ; [LKW]) \ (po ; po)