CodeSamples/count: Add necessary partial memory barrierspaulmck.2023.03.29a

This patch adds several necessary partial memory barriers, the first
one changes READ_ONCE to smp_load_acquire to make sure the reading from
theftp[t] happens before the reading from counterp[t]. The litmus testing
below represents the original code pattern, and the result is "Sometimes":

C counter_sig

{}

P0(int *theft, int *counter)
{
    int r0;
    int r1;

    r0 = READ_ONCE(*theft);
    r1 = READ_ONCE(*counter);
}

P1(int *theft, int *counter)
{
    WRITE_ONCE(*counter, 1);
    smp_mb();
    WRITE_ONCE(*theft, 1);
}

exists (0:r0=1 /\ 0:r1=0)

The second change switches from WRITE_ONCE to smp_store_release to
make sure that setting counterp[t] happens before the setting theftp[p]
to THEFT_IDLE. Here is the litmus test.  The result is "Sometimes":

C counter_sig_2

{
    int theft = 1;
    int counter = 1;
}

P0(int *theft, int *counter)
{
    WRITE_ONCE(*counter, 0);
    WRITE_ONCE(*theft, 0);
}

P1(int *theft, int *counter)
{
    if (READ_ONCE(*theft) == 0)
    {
        WRITE_ONCE(*counter, READ_ONCE(*counter)+1);
    }
}

P2(int *counter)
{
    int r0;

    r0 = READ_ONCE(*counter);
}

exists (2:r0=2)

Note that I also changed the reading of theft variable
to smp_load_acquire in add_count/sub_count's fast path
to make sure that reading theft happens before reading
counter.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/CodeSamples/count/count_lim_sig.c b/CodeSamples/count/count_lim_sig.c
index 023d6215..59da8077 100644
--- a/CodeSamples/count/count_lim_sig.c
+++ b/CodeSamples/count/count_lim_sig.c
@@ -81,7 +81,7 @@ static void flush_local_count(void)			//\lnlbl{flush:b}
 	for_each_tid(t, tid) {				//\lnlbl{flush:loop2:b}
 		if (theftp[t] == NULL)			//\lnlbl{flush:skip:nonexist}
 			continue;			//\lnlbl{flush:next2}
-		while (READ_ONCE(*theftp[t]) != THEFT_READY) {//\lnlbl{flush:loop3:b}
+		while (smp_load_acquire(theftp[t]) != THEFT_READY) {//\lnlbl{flush:loop3:b}
 			poll(NULL, 0, 1);		//\lnlbl{flush:block}
 			if (READ_ONCE(*theftp[t]) == THEFT_REQ)//\lnlbl{flush:check:REQ}
 				pthread_kill(tid, SIGUSR1);//\lnlbl{flush:signal2}
@@ -90,7 +90,7 @@ static void flush_local_count(void)			//\lnlbl{flush:b}
 		*counterp[t] = 0;
 		globalreserve -= *countermaxp[t];
 		*countermaxp[t] = 0;			//\lnlbl{flush:thiev:e}
-		WRITE_ONCE(*theftp[t], THEFT_IDLE);	//\lnlbl{flush:IDLE}
+		smp_store_release(theftp[t], THEFT_IDLE);	//\lnlbl{flush:IDLE}
 	}						//\lnlbl{flush:loop2:e}
 }							//\lnlbl{flush:e}
 
@@ -116,7 +116,7 @@ int add_count(unsigned long delta)			//\lnlbl{b}
 
 	WRITE_ONCE(counting, 1);			//\lnlbl{fast:b}
 	barrier();					//\lnlbl{barrier:1}
-	if (READ_ONCE(theft) <= THEFT_REQ &&		//\lnlbl{check:b}
+	if (smp_load_acquire(&theft) <= THEFT_REQ &&		//\lnlbl{check:b}
 	    countermax - counter >= delta) {		//\lnlbl{check:e}
 		WRITE_ONCE(counter, counter + delta);	//\lnlbl{add:f}
 		fastpath = 1;				//\lnlbl{fasttaken}
@@ -155,7 +155,7 @@ int sub_count(unsigned long delta)
 
 	WRITE_ONCE(counting, 1);
 	barrier();
-	if (READ_ONCE(theft) <= THEFT_REQ &&
+	if (smp_load_acquire(&theft) <= THEFT_REQ &&
 	    counter >= delta) {
 		WRITE_ONCE(counter, counter - delta);
 		fastpath = 1;