tools/memory-model: Add documentation about SRCU read-side critical sections

Expand the discussion of SRCU and its read-side critical sections in
the Linux Kernel Memory Model documentation file explanation.txt. The
new material discusses recent changes to the memory model made in
commit 6cd244c87428 ("tools/memory-model: Provide exact SRCU
semantics").

Signed-off-by: Alan Stern <[e-mail]>
Co-developed-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
Reviewed-by: Akira Yokosawa <[e-mail]>
Cc: Andrea Parri <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Jade Alglave <[e-mail]>
Cc: Jonas Oberhauser <[e-mail]>
Cc: Luc Maranget <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
CC: Will Deacon <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

tools/memory-model/Documentation/explanation.txt

···
   20. THE HAPPENS-BEFORE RELATION: hb
   21. THE PROPAGATES-BEFORE RELATION: pb
   22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
-  23. LOCKING
-  24. PLAIN ACCESSES AND DATA RACES
-  25. ODDS AND ENDS
+  23. SRCU READ-SIDE CRITICAL SECTIONS
+  24. LOCKING
+  25. PLAIN ACCESSES AND DATA RACES
+  26. ODDS AND ENDS
 
 
 
···
 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-order 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.
+The LKMM 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.  However, there are some significant
+differences between RCU read-side critical sections and their SRCU
+counterparts, as described in the next section.
+
+
+SRCU READ-SIDE CRITICAL SECTIONS
+--------------------------------
+
+The LKMM uses the srcu-rscsi relation to model SRCU read-side critical
+sections.  They differ from RCU read-side critical sections in the
+following respects:
+
+1.	Unlike the analogous RCU primitives, synchronize_srcu(),
+	srcu_read_lock(), and srcu_read_unlock() take a pointer to a
+	struct srcu_struct as an argument.  This structure is called
+	an SRCU domain, and calls linked by srcu-rscsi must have the
+	same domain.  Read-side critical sections and grace periods
+	associated with different domains are independent of one
+	another; the SRCU version of the RCU Guarantee applies only
+	to pairs of critical sections and grace periods having the
+	same domain.
+
+2.	srcu_read_lock() returns a value, called the index, which must
+	be passed to the matching srcu_read_unlock() call.  Unlike
+	rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
+	call does not always have to match the next unpaired
+	srcu_read_unlock().  In fact, it is possible for two SRCU
+	read-side critical sections to overlap partially, as in the
+	following example (where s is an srcu_struct and idx1 and idx2
+	are integer variables):
+
+		idx1 = srcu_read_lock(&s);	// Start of first RSCS
+		idx2 = srcu_read_lock(&s);	// Start of second RSCS
+		srcu_read_unlock(&s, idx1);	// End of first RSCS
+		srcu_read_unlock(&s, idx2);	// End of second RSCS
+
+	The matching is determined entirely by the domain pointer and
+	index value.  By contrast, if the calls had been
+	rcu_read_lock() and rcu_read_unlock() then they would have
+	created two nested (fully overlapping) read-side critical
+	sections: an inner one and an outer one.
+
+3.	The srcu_down_read() and srcu_up_read() primitives work
+	exactly like srcu_read_lock() and srcu_read_unlock(), except
+	that matching calls don't have to execute on the same CPU.
+	(The names are meant to be suggestive of operations on
+	semaphores.)  Since the matching is determined by the domain
+	pointer and index value, these primitives make it possible for
+	an SRCU read-side critical section to start on one CPU and end
+	on another, so to speak.
+
+In order to account for these properties of SRCU, the LKMM models
+srcu_read_lock() as a special type of load event (which is
+appropriate, since it takes a memory location as argument and returns
+a value, just as a load does) and srcu_read_unlock() as a special type
+of store event (again appropriate, since it takes as arguments a
+memory location and a value).  These loads and stores are annotated as
+belonging to the "srcu-lock" and "srcu-unlock" event classes
+respectively.
+
+This approach allows the LKMM to tell whether two events are
+associated with the same SRCU domain, simply by checking whether they
+access the same memory location (i.e., they are linked by the loc
+relation).  It also gives a way to tell which unlock matches a
+particular lock, by checking for the presence of a data dependency
+from the load (srcu-lock) to the store (srcu-unlock).  For example,
+given the situation outlined earlier (with statement labels added):
+
+	A: idx1 = srcu_read_lock(&s);
+	B: idx2 = srcu_read_lock(&s);
+	C: srcu_read_unlock(&s, idx1);
+	D: srcu_read_unlock(&s, idx2);
+
+the LKMM will treat A and B as loads from s yielding values saved in
+idx1 and idx2 respectively.  Similarly, it will treat C and D as
+though they stored the values from idx1 and idx2 in s.  The end result
+is much as if we had written:
+
+	A: idx1 = READ_ONCE(s);
+	B: idx2 = READ_ONCE(s);
+	C: WRITE_ONCE(s, idx1);
+	D: WRITE_ONCE(s, idx2);
+
+except for the presence of the special srcu-lock and srcu-unlock
+annotations.  You can see at once that we have A ->data C and
+B ->data D.  These dependencies tell the LKMM that C is the
+srcu-unlock event matching srcu-lock event A, and D is the
+srcu-unlock event matching srcu-lock event B.
+
+This approach is admittedly a hack, and it has the potential to lead
+to problems.  For example, in:
+
+	idx1 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx1);
+	idx2 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx2);
+
+the LKMM will believe that idx2 must have the same value as idx1,
+since it reads from the immediately preceding store of idx1 in s.
+Fortunately this won't matter, assuming that litmus tests never do
+anything with SRCU index values other than pass them to
+srcu_read_unlock() or srcu_up_read() calls.
+
+However, sometimes it is necessary to store an index value in a
+shared variable temporarily.  In fact, this is the only way for
+srcu_down_read() to pass the index it gets to an srcu_up_read() call
+on a different CPU.  In more detail, we might have soething like:
+
+	struct srcu_struct s;
+	int x;
+
+	P0()
+	{
+		int r0;
+
+		A: r0 = srcu_down_read(&s);
+		B: WRITE_ONCE(x, r0);
+	}
+
+	P1()
+	{
+		int r1;
+
+		C: r1 = READ_ONCE(x);
+		D: srcu_up_read(&s, r1);
+	}
+
+Assuming that P1 executes after P0 and does read the index value
+stored in x, we can write this (using brackets to represent event
+annotations) as:
+
+	A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
+
+The LKMM defines a carry-srcu-data relation to express this pattern;
+it permits an arbitrarily long sequence of
+
+	data ; rf
+
+pairs (that is, a data link followed by an rf link) to occur between
+an srcu-lock event and the final data dependency leading to the
+matching srcu-unlock event.  carry-srcu-data is complicated by the
+need to ensure that none of the intermediate store events in this
+sequence are instances of srcu-unlock.  This is necessary because in a
+pattern like the one above:
+
+	A: idx1 = srcu_read_lock(&s);
+	B: srcu_read_unlock(&s, idx1);
+	C: idx2 = srcu_read_lock(&s);
+	D: srcu_read_unlock(&s, idx2);
+
+the LKMM treats B as a store to the variable s and C as a load from
+that variable, creating an undesirable rf link from B to C:
+
+	A ->data B ->rf C ->data D.
+
+This would cause carry-srcu-data to mistakenly extend a data
+dependency from A to D, giving the impression that D was the
+srcu-unlock event matching A's srcu-lock.  To avoid such problems,
+carry-srcu-data does not accept sequences in which the ends of any of
+the intermediate ->data links (B above) is an srcu-unlock event.
 
 
 LOCKING