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daebf24a8e
Currently the Linux Kernel Memory Model gives an incorrect response
for the following litmus test:
C plain-WWC
{}
P0(int *x)
{
WRITE_ONCE(*x, 2);
}
P1(int *x, int *y)
{
int r1;
int r2;
int r3;
r1 = READ_ONCE(*x);
if (r1 == 2) {
smp_rmb();
r2 = *x;
}
smp_rmb();
r3 = READ_ONCE(*x);
WRITE_ONCE(*y, r3 - 1);
}
P2(int *x, int *y)
{
int r4;
r4 = READ_ONCE(*y);
if (r4 > 0)
WRITE_ONCE(*x, 1);
}
exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
The memory model says that the plain read of *x in P1 races with the
WRITE_ONCE(*x) in P2.
The problem is that we have a write W and a read R related by neither
fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
write (the WRITE_ONCE() in P0). In this situation there is no
particular ordering between W and R, so either a wr-vis link from W to
R or an rw-xbstar link from R to W would prove that the accesses
aren't concurrent.
But the LKMM only looks for a wr-vis link, which is equivalent to
assuming that W must execute before R. This is not necessarily true
on non-multicopy-atomic systems, as the WWC pattern demonstrates.
This patch changes the LKMM to accept either a wr-vis or a reverse
rw-xbstar link as a proof of non-concurrency.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
204 lines
7.1 KiB
Plaintext
204 lines
7.1 KiB
Plaintext
// SPDX-License-Identifier: GPL-2.0+
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(*
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* Copyright (C) 2015 Jade Alglave <j.alglave@ucl.ac.uk>,
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* Copyright (C) 2016 Luc Maranget <luc.maranget@inria.fr> for Inria
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* Copyright (C) 2017 Alan Stern <stern@rowland.harvard.edu>,
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* Andrea Parri <parri.andrea@gmail.com>
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*
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* An earlier version of this file appeared in the companion webpage for
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* "Frightening small children and disconcerting grown-ups: Concurrency
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* in the Linux kernel" by Alglave, Maranget, McKenney, Parri, and Stern,
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* which appeared in ASPLOS 2018.
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*)
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"Linux-kernel memory consistency model"
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(*
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* File "lock.cat" handles locks and is experimental.
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* It can be replaced by include "cos.cat" for tests that do not use locks.
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*)
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include "lock.cat"
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(*******************)
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(* Basic relations *)
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(*******************)
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(* Release Acquire *)
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let acq-po = [Acquire] ; po ; [M]
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let po-rel = [M] ; po ; [Release]
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let po-unlock-rf-lock-po = po ; [UL] ; rf ; [LKR] ; po
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(* Fences *)
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let R4rmb = R \ Noreturn (* Reads for which rmb works *)
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let rmb = [R4rmb] ; fencerel(Rmb) ; [R4rmb]
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let wmb = [W] ; fencerel(Wmb) ; [W]
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let mb = ([M] ; fencerel(Mb) ; [M]) |
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([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
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([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
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([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
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([M] ; po ; [UL] ; (co | po) ; [LKW] ;
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fencerel(After-unlock-lock) ; [M])
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let gp = po ; [Sync-rcu | Sync-srcu] ; po?
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let strong-fence = mb | gp
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let nonrw-fence = strong-fence | po-rel | acq-po
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let fence = nonrw-fence | wmb | rmb
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let barrier = fencerel(Barrier | Rmb | Wmb | Mb | Sync-rcu | Sync-srcu |
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Before-atomic | After-atomic | Acquire | Release |
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Rcu-lock | Rcu-unlock | Srcu-lock | Srcu-unlock) |
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(po ; [Release]) | ([Acquire] ; po)
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(**********************************)
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(* Fundamental coherence ordering *)
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(**********************************)
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(* Sequential Consistency Per Variable *)
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let com = rf | co | fr
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acyclic po-loc | com as coherence
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(* Atomic Read-Modify-Write *)
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empty rmw & (fre ; coe) as atomic
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(**********************************)
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(* Instruction execution ordering *)
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(**********************************)
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(* Preserved Program Order *)
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let dep = addr | data
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let rwdep = (dep | ctrl) ; [W]
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let overwrite = co | fr
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let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
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let to-r = addr | (dep ; [Marked] ; rfi)
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let ppo = to-r | to-w | fence | (po-unlock-rf-lock-po & int)
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(* Propagation: Ordering from release operations and strong fences. *)
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let A-cumul(r) = (rfe ; [Marked])? ; r
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let cumul-fence = [Marked] ; (A-cumul(strong-fence | po-rel) | wmb |
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po-unlock-rf-lock-po) ; [Marked]
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let prop = [Marked] ; (overwrite & ext)? ; cumul-fence* ;
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[Marked] ; rfe? ; [Marked]
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(*
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* Happens Before: Ordering from the passage of time.
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* No fences needed here for prop because relation confined to one process.
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*)
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let hb = [Marked] ; (ppo | rfe | ((prop \ id) & int)) ; [Marked]
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acyclic hb as happens-before
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(****************************************)
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(* Write and fence propagation ordering *)
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(****************************************)
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(* Propagation: Each non-rf link needs a strong fence. *)
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let pb = prop ; strong-fence ; hb* ; [Marked]
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acyclic pb as propagation
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(*******)
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(* RCU *)
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(*******)
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(*
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* Effects of read-side critical sections proceed from the rcu_read_unlock()
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* or srcu_read_unlock() backwards on the one hand, and from the
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* rcu_read_lock() or srcu_read_lock() forwards on the other hand.
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*
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* In the definition of rcu-fence below, the po term at the left-hand side
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* of each disjunct and the po? term at the right-hand end have been factored
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* out. They have been moved into the definitions of rcu-link and rb.
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* This was necessary in order to apply the "& loc" tests correctly.
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*)
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let rcu-gp = [Sync-rcu] (* Compare with gp *)
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let srcu-gp = [Sync-srcu]
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let rcu-rscsi = rcu-rscs^-1
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let srcu-rscsi = srcu-rscs^-1
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(*
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* The synchronize_rcu() strong fence is special in that it can order not
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* one but two non-rf relations, but only in conjunction with an RCU
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* read-side critical section.
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*)
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let rcu-link = po? ; hb* ; pb* ; prop ; po
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(*
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* Any sequence containing at least as many grace periods as RCU read-side
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* critical sections (joined by rcu-link) induces order like a generalized
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* inter-CPU strong fence.
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* Likewise for SRCU grace periods and read-side critical sections, provided
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* the synchronize_srcu() and srcu_read_[un]lock() calls refer to the same
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* struct srcu_struct location.
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*)
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let rec rcu-order = rcu-gp | srcu-gp |
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(rcu-gp ; rcu-link ; rcu-rscsi) |
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((srcu-gp ; rcu-link ; srcu-rscsi) & loc) |
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(rcu-rscsi ; rcu-link ; rcu-gp) |
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((srcu-rscsi ; rcu-link ; srcu-gp) & loc) |
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(rcu-gp ; rcu-link ; rcu-order ; rcu-link ; rcu-rscsi) |
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((srcu-gp ; rcu-link ; rcu-order ; rcu-link ; srcu-rscsi) & loc) |
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(rcu-rscsi ; rcu-link ; rcu-order ; rcu-link ; rcu-gp) |
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((srcu-rscsi ; rcu-link ; rcu-order ; rcu-link ; srcu-gp) & loc) |
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(rcu-order ; rcu-link ; rcu-order)
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let rcu-fence = po ; rcu-order ; po?
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let fence = fence | rcu-fence
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let strong-fence = strong-fence | rcu-fence
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(* rb orders instructions just as pb does *)
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let rb = prop ; rcu-fence ; hb* ; pb* ; [Marked]
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irreflexive rb as rcu
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(*
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* The happens-before, propagation, and rcu constraints are all
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* expressions of temporal ordering. They could be replaced by
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* a single constraint on an "executes-before" relation, xb:
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*
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* let xb = hb | pb | rb
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* acyclic xb as executes-before
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*)
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(*********************************)
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(* Plain accesses and data races *)
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(*********************************)
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(* Warn about plain writes and marked accesses in the same region *)
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let mixed-accesses = ([Plain & W] ; (po-loc \ barrier) ; [Marked]) |
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([Marked] ; (po-loc \ barrier) ; [Plain & W])
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flag ~empty mixed-accesses as mixed-accesses
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(* Executes-before and visibility *)
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let xbstar = (hb | pb | rb)*
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let vis = cumul-fence* ; rfe? ; [Marked] ;
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((strong-fence ; [Marked] ; xbstar) | (xbstar & int))
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(* Boundaries for lifetimes of plain accesses *)
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let w-pre-bounded = [Marked] ; (addr | fence)?
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let r-pre-bounded = [Marked] ; (addr | nonrw-fence |
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([R4rmb] ; fencerel(Rmb) ; [~Noreturn]))?
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let w-post-bounded = fence? ; [Marked]
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let r-post-bounded = (nonrw-fence | ([~Noreturn] ; fencerel(Rmb) ; [R4rmb]))? ;
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[Marked]
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(* Visibility and executes-before for plain accesses *)
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let ww-vis = fence | (strong-fence ; xbstar ; w-pre-bounded) |
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(w-post-bounded ; vis ; w-pre-bounded)
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let wr-vis = fence | (strong-fence ; xbstar ; r-pre-bounded) |
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(w-post-bounded ; vis ; r-pre-bounded)
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let rw-xbstar = fence | (r-post-bounded ; xbstar ; w-pre-bounded)
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(* Potential races *)
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let pre-race = ext & ((Plain * M) | ((M \ IW) * Plain))
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(* Coherence requirements for plain accesses *)
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let wr-incoh = pre-race & rf & rw-xbstar^-1
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let rw-incoh = pre-race & fr & wr-vis^-1
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let ww-incoh = pre-race & co & ww-vis^-1
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empty (wr-incoh | rw-incoh | ww-incoh) as plain-coherence
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(* Actual races *)
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let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
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let ww-race = (pre-race & co) \ ww-nonrace
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let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
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let rw-race = (pre-race & fr) \ rw-xbstar
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flag ~empty (ww-race | wr-race | rw-race) as data-race
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