diff options
author | Alan Huang <mmpgouride@gmail.com> | 2023-03-28 10:58:24 -0400 |
---|---|---|
committer | Paul E. McKenney <paulmck@kernel.org> | 2023-03-29 16:21:02 -0700 |
commit | c72371753921a6f986f6f3385fbf814e1276be3d (patch) | |
tree | 92dacdb92f489e08b1544b96e7bfcfbf994b3cdb | |
parent | a7dcda34850ac783b20927699e886f207676ea7a (diff) | |
download | perfbook-paulmck.2023.03.29a.tar.gz |
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>
-rw-r--r-- | CodeSamples/count/count_lim_sig.c | 8 |
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; |