Re: [PATCH] tools/memory-model: Update Documentation/explanation.txt to include SRCU support

[Paul E. McKenney]

On Tue, Dec 11, 2018 at 11:38:53AM -0500, Alan Stern wrote:
> The recent commit adding support for SRCU to the Linux Kernel Memory
> Model ended up changing the names and meanings of several relations.
> This patch updates the explanation.txt documentation file to reflect
> those changes.
> 
> It also revises the statement of the RCU Guarantee to a more accurate
> form, and it adds a short paragraph mentioning the new support for SRCU.
> 
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Cc: Akira Yokosawa <akiyks@xxxxxxxxx>
> Cc: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Daniel Lustig <dlustig@xxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Jade Alglave <j.alglave@xxxxxxxxx>
> Cc: Luc Maranget <luc.maranget@xxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>

Queued and pushed, thank you!

Would anyone be willing to ack/review/whatever?

							Thanx, Paul

> ---
> 
> 
>  tools/memory-model/Documentation/explanation.txt |  293 ++++++++++++-----------
>  1 file changed, 154 insertions(+), 139 deletions(-)
> 
> Index: usb-4.x/tools/memory-model/Documentation/explanation.txt
> ===================================================================
> --- usb-4.x.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-4.x/tools/memory-model/Documentation/explanation.txt
> @@ -27,7 +27,7 @@ Explanation of the Linux-Kernel Memory C
>    19. AND THEN THERE WAS ALPHA
>    20. THE HAPPENS-BEFORE RELATION: hb
>    21. THE PROPAGATES-BEFORE RELATION: pb
> -  22. RCU RELATIONS: rcu-link, gp, rscs, rcu-fence, and rb
> +  22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-fence, and rb
>    23. LOCKING
>    24. ODDS AND ENDS
>  
> @@ -1430,8 +1430,8 @@ they execute means that it cannot have c
>  the content of the LKMM's "propagation" axiom.
>  
>  
> -RCU RELATIONS: rcu-link, gp, rscs, rcu-fence, and rb
> -----------------------------------------------------
> +RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-fence, and rb
> +-------------------------------------------------------------
>  
>  RCU (Read-Copy-Update) is a powerful synchronization mechanism.  It
>  rests on two concepts: grace periods and read-side critical sections.
> @@ -1446,17 +1446,19 @@ As far as memory models are concerned, R
>  Grace-Period Guarantee, which states that a critical section can never
>  span a full grace period.  In more detail, the Guarantee says:
>  
> -	If a critical section starts before a grace period then it
> -	must end before the grace period does.  In addition, every
> -	store that propagates to the critical section's CPU before the
> -	end of the critical section must propagate to every CPU before
> -	the end of the grace period.
> -
> -	If a critical section ends after a grace period ends then it
> -	must start after the grace period does.  In addition, every
> -	store that propagates to the grace period's CPU before the
> -	start of the grace period must propagate to every CPU before
> -	the start of the critical section.
> +	For any critical section C and any grace period G, at least
> +	one of the following statements must hold:
> +
> +(1)	C ends before G does, and in addition, every store that
> +	propagates to C's CPU before the end of C must propagate to
> +	every CPU before G ends.
> +
> +(2)	G starts before C does, and in addition, every store that
> +	propagates to G's CPU before the start of G must propagate
> +	to every CPU before C starts.
> +
> +In particular, it is not possible for a critical section to both start
> +before and end after a grace period.
>  
>  Here is a simple example of RCU in action:
>  
> @@ -1483,10 +1485,11 @@ The Grace Period Guarantee tells us that
>  never end with r1 = 1 and r2 = 0.  The reasoning is as follows.  r1 = 1
>  means that P0's store to x propagated to P1 before P1 called
>  synchronize_rcu(), so P0's critical section must have started before
> -P1's grace period.  On the other hand, r2 = 0 means that P0's store to
> -y, which occurs before the end of the critical section, did not
> -propagate to P1 before the end of the grace period, violating the
> -Guarantee.
> +P1's grace period, contrary to part (2) of the Guarantee.  On the
> +other hand, r2 = 0 means that P0's store to y, which occurs before the
> +end of the critical section, did not propagate to P1 before the end of
> +the grace period, contrary to part (1).  Together the results violate
> +the Guarantee.
>  
>  In the kernel's implementations of RCU, the requirements for stores
>  to propagate to every CPU are fulfilled by placing strong fences at
> @@ -1504,11 +1507,11 @@ before" or "ends after" a grace period?
>  are pretty obvious, as in the example above, but the details aren't
>  entirely clear.  The LKMM formalizes this notion by means of the
>  rcu-link relation.  rcu-link encompasses a very general notion of
> -"before": Among other things, X ->rcu-link Z includes cases where X
> -happens-before or is equal to some event Y which is equal to or comes
> -before Z in the coherence order.  When Y = Z this says that X ->rfe Z
> -implies X ->rcu-link Z.  In addition, when Y = X it says that X ->fr Z
> -and X ->co Z each imply X ->rcu-link Z.
> +"before": If E and F are RCU fence events (i.e., rcu_read_lock(),
> +rcu_read_unlock(), or synchronize_rcu()) then among other things,
> +E ->rcu-link F includes cases where E is po-before some memory-access
> +event X, F is po-after some memory-access event Y, and we have any of
> +X ->rfe Y, X ->co Y, or X ->fr Y.
>  
>  The formal definition of the rcu-link relation is more than a little
>  obscure, and we won't give it here.  It is closely related to the pb
> @@ -1516,171 +1519,173 @@ relation, and the details don't matter u
>  a somewhat lengthy formal proof.  Pretty much all you need to know
>  about rcu-link is the information in the preceding paragraph.
>  
> -The LKMM also defines the gp and rscs relations.  They bring grace
> -periods and read-side critical sections into the picture, in the
> +The LKMM also defines the rcu-gp and rcu-rscsi relations.  They bring
> +grace periods and read-side critical sections into the picture, in the
>  following way:
>  
> -	E ->gp F means there is a synchronize_rcu() fence event S such
> -	that E ->po S and either S ->po F or S = F.  In simple terms,
> -	there is a grace period po-between E and F.
> -
> -	E ->rscs F means there is a critical section delimited by an
> -	rcu_read_lock() fence L and an rcu_read_unlock() fence U, such
> -	that E ->po U and either L ->po F or L = F.  You can think of
> -	this as saying that E and F are in the same critical section
> -	(in fact, it also allows E to be po-before the start of the
> -	critical section and F to be po-after the end).
> +	E ->rcu-gp F means that E and F are in fact the same event,
> +	and that event is a synchronize_rcu() fence (i.e., a grace
> +	period).
> +
> +	E ->rcu-rscsi F means that E and F are the rcu_read_unlock()
> +	and rcu_read_lock() fence events delimiting some read-side
> +	critical section.  (The 'i' at the end of the name emphasizes
> +	that this relation is "inverted": It links the end of the
> +	critical section to the start.)
>  
>  If we think of the rcu-link relation as standing for an extended
> -"before", then X ->gp Y ->rcu-link Z says that X executes before a
> -grace period which ends before Z executes.  (In fact it covers more
> -than this, because it also includes cases where X executes before a
> -grace period and some store propagates to Z's CPU before Z executes
> -but doesn't propagate to some other CPU until after the grace period
> -ends.)  Similarly, X ->rscs Y ->rcu-link Z says that X is part of (or
> -before the start of) a critical section which starts before Z
> -executes.
> -
> -The LKMM goes on to define the rcu-fence relation as a sequence of gp
> -and rscs links separated by rcu-link links, in which the number of gp
> -links is >= the number of rscs links.  For example:
> +"before", then X ->rcu-gp Y ->rcu-link Z roughly says that X is a
> +grace period which ends before Z begins.  (In fact it covers more than
> +this, because it also includes cases where some store propagates to
> +Z's CPU before Z begins but doesn't propagate to some other CPU until
> +after X ends.)  Similarly, X ->rcu-rscsi Y ->rcu-link Z says that X is
> +the end of a critical section which starts before Z begins.
> +
> +The LKMM goes on to define the rcu-fence relation as a sequence of
> +rcu-gp and rcu-rscsi links separated by rcu-link links, in which the
> +number of rcu-gp links is >= the number of rcu-rscsi links.  For
> +example:
>  
> -	X ->gp Y ->rcu-link Z ->rscs T ->rcu-link U ->gp V
> +	X ->rcu-gp Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V
>  
>  would imply that X ->rcu-fence V, because this sequence contains two
> -gp links and only one rscs link.  (It also implies that X ->rcu-fence T
> -and Z ->rcu-fence V.)  On the other hand:
> +rcu-gp links and one rcu-rscsi link.  (It also implies that
> +X ->rcu-fence T and Z ->rcu-fence V.)  On the other hand:
>  
> -	X ->rscs Y ->rcu-link Z ->rscs T ->rcu-link U ->gp V
> +	X ->rcu-rscsi Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V
>  
>  does not imply X ->rcu-fence V, because the sequence contains only
> -one gp link but two rscs links.
> +one rcu-gp link but two rcu-rscsi links.
>  
>  The rcu-fence relation is important because the Grace Period Guarantee
>  means that rcu-fence acts kind of like a strong fence.  In particular,
> -if W is a write and we have W ->rcu-fence Z, the Guarantee says that W
> -will propagate to every CPU before Z executes.
> +E ->rcu-fence F implies not only that E begins before F ends, but also
> +that any write po-before E will propagate to every CPU before any
> +instruction po-after F can execute.  (However, it does not imply that
> +E must execute before F; in fact, each synchronize_rcu() fence event
> +is linked to itself by rcu-fence as a degenerate case.)
>  
>  To prove this in full generality requires some intellectual effort.
>  We'll consider just a very simple case:
>  
> -	W ->gp X ->rcu-link Y ->rscs Z.
> +	G ->rcu-gp W ->rcu-link Z ->rcu-rscsi F.
>  
> -This formula means that there is a grace period G and a critical
> -section C such that:
> +This formula means that G and W are the same event (a grace period),
> +and there are events X, Y and a read-side critical section C such that:
>  
> -	1. W is po-before G;
> +	1. G = W is po-before or equal to X;
>  
> -	2. X is equal to or po-after G;
> +	2. X comes "before" Y in some sense (including rfe, co and fr);
>  
> -	3. X comes "before" Y in some sense;
> +	2. Y is po-before Z;
>  
> -	4. Y is po-before the end of C;
> +	4. Z is the rcu_read_unlock() event marking the end of C;
>  
> -	5. Z is equal to or po-after the start of C.
> +	5. F is the rcu_read_lock() event marking the start of C.
>  
> -From 2 - 4 we deduce that the grace period G ends before the critical
> -section C.  Then the second part of the Grace Period Guarantee says
> -not only that G starts before C does, but also that W (which executes
> -on G's CPU before G starts) must propagate to every CPU before C
> -starts.  In particular, W propagates to every CPU before Z executes
> -(or finishes executing, in the case where Z is equal to the
> -rcu_read_lock() fence event which starts C.)  This sort of reasoning
> -can be expanded to handle all the situations covered by rcu-fence.
> +From 1 - 4 we deduce that the grace period G ends before the critical
> +section C.  Then part (2) of the Grace Period Guarantee says not only
> +that G starts before C does, but also that any write which executes on
> +G's CPU before G starts must propagate to every CPU before C starts.
> +In particular, the write propagates to every CPU before F finishes
> +executing and hence before any instruction po-after F can execute.
> +This sort of reasoning can be extended to handle all the situations
> +covered by rcu-fence.
>  
>  Finally, the LKMM defines the RCU-before (rb) relation in terms of
>  rcu-fence.  This is done in essentially the same way as the pb
>  relation was defined in terms of strong-fence.  We will omit the
> -details; the end result is that E ->rb F implies E must execute before
> -F, just as E ->pb F does (and for much the same reasons).
> +details; the end result is that E ->rb F implies E must execute
> +before F, just as E ->pb F does (and for much the same reasons).
>  
>  Putting this all together, the LKMM expresses the Grace Period
>  Guarantee by requiring that the rb relation does not contain a cycle.
> -Equivalently, this "rcu" axiom requires that there are no events E and
> -F with E ->rcu-link F ->rcu-fence E.  Or to put it a third way, the
> -axiom requires that there are no cycles consisting of gp and rscs
> -alternating with rcu-link, where the number of gp links is >= the
> -number of rscs links.
> +Equivalently, this "rcu" axiom requires that there are no events E
> +and F with E ->rcu-link F ->rcu-fence E.  Or to put it a third way,
> +the axiom requires that there are no cycles consisting of rcu-gp and
> +rcu-rscsi alternating with rcu-link, where the number of rcu-gp links
> +is >= the number of rcu-rscsi links.
>  
>  Justifying the axiom isn't easy, but it is in fact a valid
>  formalization of the Grace Period Guarantee.  We won't attempt to go
>  through the detailed argument, but the following analysis gives a
> -taste of what is involved.  Suppose we have a violation of the first
> -part of the Guarantee: A critical section starts before a grace
> -period, and some store propagates to the critical section's CPU before
> -the end of the critical section but doesn't propagate to some other
> -CPU until after the end of the grace period.
> +taste of what is involved.  Suppose both parts of the Guarantee are
> +violated: A critical section starts before a grace period, and some
> +store propagates to the critical section's CPU before the end of the
> +critical section but doesn't propagate to some other CPU until after
> +the end of the grace period.
>  
>  Putting symbols to these ideas, let L and U be the rcu_read_lock() and
>  rcu_read_unlock() fence events delimiting the critical section in
>  question, and let S be the synchronize_rcu() fence event for the grace
>  period.  Saying that the critical section starts before S means there
> -are events E and F where E is po-after L (which marks the start of the
> -critical section), E is "before" F in the sense of the rcu-link
> -relation, and F is po-before the grace period S:
> +are events Q and R where Q is po-after L (which marks the start of the
> +critical section), Q is "before" R in the sense used by the rcu-link
> +relation, and R is po-before the grace period S.  Thus we have:
>  
> -	L ->po E ->rcu-link F ->po S.
> +	L ->rcu-link S.
>  
> -Let W be the store mentioned above, let Z come before the end of the
> +Let W be the store mentioned above, let Y come before the end of the
>  critical section and witness that W propagates to the critical
> -section's CPU by reading from W, and let Y on some arbitrary CPU be a
> -witness that W has not propagated to that CPU, where Y happens after
> +section's CPU by reading from W, and let Z on some arbitrary CPU be a
> +witness that W has not propagated to that CPU, where Z happens after
>  some event X which is po-after S.  Symbolically, this amounts to:
>  
> -	S ->po X ->hb* Y ->fr W ->rf Z ->po U.
> +	S ->po X ->hb* Z ->fr W ->rf Y ->po U.
>  
> -The fr link from Y to W indicates that W has not propagated to Y's CPU
> -at the time that Y executes.  From this, it can be shown (see the
> -discussion of the rcu-link relation earlier) that X and Z are related
> -by rcu-link, yielding:
> +The fr link from Z to W indicates that W has not propagated to Z's CPU
> +at the time that Z executes.  From this, it can be shown (see the
> +discussion of the rcu-link relation earlier) that S and U are related
> +by rcu-link:
>  
> -	S ->po X ->rcu-link Z ->po U.
> +	S ->rcu-link U.
>  
> -The formulas say that S is po-between F and X, hence F ->gp X.  They
> -also say that Z comes before the end of the critical section and E
> -comes after its start, hence Z ->rscs E.  From all this we obtain:
> +Since S is a grace period we have S ->rcu-gp S, and since L and U are
> +the start and end of the critical section C we have U ->rcu-rscsi L.
> +From this we obtain:
>  
> -	F ->gp X ->rcu-link Z ->rscs E ->rcu-link F,
> +	S ->rcu-gp S ->rcu-link U ->rcu-rscsi L ->rcu-link S,
>  
>  a forbidden cycle.  Thus the "rcu" axiom rules out this violation of
>  the Grace Period Guarantee.
>  
>  For something a little more down-to-earth, let's see how the axiom
>  works out in practice.  Consider the RCU code example from above, this
> -time with statement labels added to the memory access instructions:
> +time with statement labels added:
>  
>  	int x, y;
>  
>  	P0()
>  	{
> -		rcu_read_lock();
> -		W: WRITE_ONCE(x, 1);
> -		X: WRITE_ONCE(y, 1);
> -		rcu_read_unlock();
> +		L: rcu_read_lock();
> +		X: WRITE_ONCE(x, 1);
> +		Y: WRITE_ONCE(y, 1);
> +		U: rcu_read_unlock();
>  	}
>  
>  	P1()
>  	{
>  		int r1, r2;
>  
> -		Y: r1 = READ_ONCE(x);
> -		synchronize_rcu();
> -		Z: r2 = READ_ONCE(y);
> +		Z: r1 = READ_ONCE(x);
> +		S: synchronize_rcu();
> +		W: r2 = READ_ONCE(y);
>  	}
>  
>  
> -If r2 = 0 at the end then P0's store at X overwrites the value that
> -P1's load at Z reads from, so we have Z ->fre X and thus Z ->rcu-link X.
> -In addition, there is a synchronize_rcu() between Y and Z, so therefore
> -we have Y ->gp Z.
> +If r2 = 0 at the end then P0's store at Y overwrites the value that
> +P1's load at W reads from, so we have W ->fre Y.  Since S ->po W and
> +also Y ->po U, we get S ->rcu-link U.  In addition, S ->rcu-gp S
> +because S is a grace period.
>  
> -If r1 = 1 at the end then P1's load at Y reads from P0's store at W,
> -so we have W ->rcu-link Y.  In addition, W and X are in the same critical
> -section, so therefore we have X ->rscs W.
> +If r1 = 1 at the end then P1's load at Z reads from P0's store at X,
> +so we have X ->rfe Z.  Together with L ->po X and Z ->po S, this
> +yields L ->rcu-link S.  And since L and U are the start and end of a
> +critical section, we have U ->rcu-rscsi L.
>  
> -Then X ->rscs W ->rcu-link Y ->gp Z ->rcu-link X is a forbidden cycle,
> -violating the "rcu" axiom.  Hence the outcome is not allowed by the
> -LKMM, as we would expect.
> +Then U ->rcu-rscsi L ->rcu-link S ->rcu-gp S ->rcu-link U is a
> +forbidden cycle, violating the "rcu" axiom.  Hence the outcome is not
> +allowed by the LKMM, as we would expect.
>  
>  For contrast, let's see what can happen in a more complicated example:
>  
> @@ -1690,51 +1695,52 @@ For contrast, let's see what can happen
>  	{
>  		int r0;
>  
> -		rcu_read_lock();
> -		W: r0 = READ_ONCE(x);
> -		X: WRITE_ONCE(y, 1);
> -		rcu_read_unlock();
> +		L0: rcu_read_lock();
> +		    r0 = READ_ONCE(x);
> +		    WRITE_ONCE(y, 1);
> +		U0: rcu_read_unlock();
>  	}
>  
>  	P1()
>  	{
>  		int r1;
>  
> -		Y: r1 = READ_ONCE(y);
> -		synchronize_rcu();
> -		Z: WRITE_ONCE(z, 1);
> +		    r1 = READ_ONCE(y);
> +		S1: synchronize_rcu();
> +		    WRITE_ONCE(z, 1);
>  	}
>  
>  	P2()
>  	{
>  		int r2;
>  
> -		rcu_read_lock();
> -		U: r2 = READ_ONCE(z);
> -		V: WRITE_ONCE(x, 1);
> -		rcu_read_unlock();
> +		L2: rcu_read_lock();
> +		    r2 = READ_ONCE(z);
> +		    WRITE_ONCE(x, 1);
> +		U2: rcu_read_unlock();
>  	}
>  
>  If r0 = r1 = r2 = 1 at the end, then similar reasoning to before shows
> -that W ->rscs X ->rcu-link Y ->gp Z ->rcu-link U ->rscs V ->rcu-link W.
> -However this cycle is not forbidden, because the sequence of relations
> -contains fewer instances of gp (one) than of rscs (two).  Consequently
> -the outcome is allowed by the LKMM.  The following instruction timing
> -diagram shows how it might actually occur:
> +that U0 ->rcu-rscsi L0 ->rcu-link S1 ->rcu-gp S1 ->rcu-link U2 ->rcu-rscsi
> +L2 ->rcu-link U0.  However this cycle is not forbidden, because the
> +sequence of relations contains fewer instances of rcu-gp (one) than of
> +rcu-rscsi (two).  Consequently the outcome is allowed by the LKMM.
> +The following instruction timing diagram shows how it might actually
> +occur:
>  
>  P0			P1			P2
>  --------------------	--------------------	--------------------
>  rcu_read_lock()
> -X: WRITE_ONCE(y, 1)
> -			Y: r1 = READ_ONCE(y)
> +WRITE_ONCE(y, 1)
> +			r1 = READ_ONCE(y)
>  			synchronize_rcu() starts
>  			.			rcu_read_lock()
> -			.			V: WRITE_ONCE(x, 1)
> -W: r0 = READ_ONCE(x)	.
> +			.			WRITE_ONCE(x, 1)
> +r0 = READ_ONCE(x)	.
>  rcu_read_unlock()	.
>  			synchronize_rcu() ends
> -			Z: WRITE_ONCE(z, 1)
> -						U: r2 = READ_ONCE(z)
> +			WRITE_ONCE(z, 1)
> +						r2 = READ_ONCE(z)
>  						rcu_read_unlock()
>  
>  This requires P0 and P2 to execute their loads and stores out of
> @@ -1744,6 +1750,15 @@ section in P0 both starts before P1's gr
>  before it does, and the critical section in P2 both starts after P1's
>  grace period does and ends after it does.
>  
> +Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
> +addition to normal RCU.  The ideas involved are much the same as
> +above, with new relations srcu-gp and srcu-rscsi added to represent
> +SRCU grace periods and read-side critical sections.  There is a
> +restriction on the srcu-gp and srcu-rscsi links that can appear in an
> +rcu-fence sequence (the srcu-rscsi links must be paired with srcu-gp
> +links having the same SRCU domain with proper nesting); the details
> +are relatively unimportant.
> +
>  
>  LOCKING
>  -------
>