Messages in this thread | | | Date | Fri, 13 Jan 2023 12:32:41 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Fri, Jan 13, 2023 at 12:07:06PM -0800, Paul E. McKenney wrote: > On Fri, Jan 13, 2023 at 11:28:10AM -0500, Alan Stern wrote: > > On Thu, Jan 12, 2023 at 01:48:26PM +0000, Jonas Oberhauser wrote: > > > From: Alan Stern [mailto:stern@rowland.harvard.edu] > > > Sent: Wednesday, January 11, 2023 4:06 PM
[ . . . ]
> > SRCU is exactly like RCU except for one aspect: The SRCU primitives > > (synchronize_srcu(), srcu_lock(), and srcu_unlock()) each take an > > argument, a pointer to an srcu structure. The ordering restrictions > > apply only in cases where the arguments to the corresponding > > primitives point to the _same_ srcu structure. That's why you see all > > those "& loc" expressions sprinkled throughout the definitions of > > srcu-rscs and rcu-order. > > In addition, the actual Linux-kernel SRCU has srcu_read_lock() return a > value that must be passed to srcu_read_unlock(). This means that SRCU > can have distinct overlapping SRCU read-side critical sections within > the confines of a given process. > > Worse yet, the upcoming addition of srcu_down_read() and srcu_up_read() > means that a given SRCU read-side critical section might begin on one > process and end on another. Thus srcu_down_read() is to srcu_read_lock() > as down_sema() is to mutex_lock(), more or less. > > Making LKMM correctly model all of this has been on my todo list for an > embarrassingly long time.
But there is no time like the present...
Here is what mainline has to recognize SRCU read-side critical sections:
------------------------------------------------------------------------
(* 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
(* 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
------------------------------------------------------------------------
And here is what I just now tried:
------------------------------------------------------------------------
(* Compute matching pairs of Srcu-lock and Srcu-unlock *) let srcu-rscs = ([Srcu-lock] ; rfi ; [Srcu-unlock]) & loc
(* Validate nesting *) flag empty srcu-rscs as no-srcu-readers 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
------------------------------------------------------------------------
This gets me "Flag no-srcu-readers" when running this litmus test:
------------------------------------------------------------------------
C C-srcu-nest-1
(* * Result: Never *)
{}
P0(int *x, int *y, struct srcu_struct *s) { int r1; int r2; int r3;
r3 = srcu_read_lock(s); r1 = READ_ONCE(*x); srcu_read_unlock(s, r3); r3 = srcu_read_lock(s); r2 = READ_ONCE(*y); srcu_read_unlock(s, r3); }
P1(int *x, int *y, struct srcu_struct *s) { WRITE_ONCE(*y, 1); synchronize_srcu(s); WRITE_ONCE(*x, 1); }
locations [0:r1] exists (0:r1=1 /\ 0:r2=0)
------------------------------------------------------------------------
So what did I mess up this time? ;-)
Thanx, Paul
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