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 <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Jade Alglave <j.alglave@xxxxxxxxx>
Cc: Luc Maranget <luc.maranget@xxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxx>
---
Joel, please feel free to add your Co-developed-by and Signed-off-by
tags to this patch.
tools/memory-model/Documentation/explanation.txt | 178 +++++++++++++++++++++--
1 file changed, 167 insertions(+), 11 deletions(-)
Index: usb-devel/tools/memory-model/Documentation/explanation.txt
===================================================================
--- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
+++ usb-devel/tools/memory-model/Documentation/explanation.txt
@@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
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
@@ -1848,14 +1849,169 @@ 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-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 important
+differences between RCU read-side critical sections and their SRCU
+counterparts, as described in the next section.
+
+
+SRCU READ-SIDE CRITICAL SECTIONS
+--------------------------------
+
+The LKMM models uses the srcu-rscsi relation to model SRCU read-side
+critical sections. They are different 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
