[tip: core/rcu] scftorture: Prevent compiler from reducing race probabilities

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     ee7035d29576dcb59b1191e5f609517cacab1e56
Gitweb:        
https://git.kernel.org/tip/ee7035d29576dcb59b1191e5f609517cacab1e56
Author:        Paul E. McKenney <paul...@kernel.org>
AuthorDate:    Wed, 01 Jul 2020 16:38:16 -07:00
Committer:     Paul E. McKenney <paul...@kernel.org>
CommitterDate: Mon, 24 Aug 2020 18:38:36 -07:00

scftorture: Prevent compiler from reducing race probabilities

Detecting smp_call_function() memory misordering requires close timing,
so it is necessary to have the checks immediately before and after
the call to the smp_call_function*() function under test.  This commit
therefore inserts barrier() calls to prevent the compiler from optimizing
memory-misordering detection down into the zone of extreme improbability.

Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
 kernel/scftorture.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/scftorture.c b/kernel/scftorture.c
index 880c2ce..8349681 100644
--- a/kernel/scftorture.c
+++ b/kernel/scftorture.c
@@ -322,6 +322,7 @@ static void scftorture_invoke_one(struct scf_statistics 
*scfp, struct torture_ra
                        scfp->n_single++;
                if (scfcp) {
                        scfcp->scfc_cpu = cpu;
+                       barrier(); // Prevent race-reduction compiler 
optimizations.
                        scfcp->scfc_in = true;
                }
                ret = smp_call_function_single(cpu, scf_handler_1, (void 
*)scfcp, scfsp->scfs_wait);
@@ -339,8 +340,10 @@ static void scftorture_invoke_one(struct scf_statistics 
*scfp, struct torture_ra
                        scfp->n_many_wait++;
                else
                        scfp->n_many++;
-               if (scfcp)
+               if (scfcp) {
+                       barrier(); // Prevent race-reduction compiler 
optimizations.
                        scfcp->scfc_in = true;
+               }
                smp_call_function_many(cpu_online_mask, scf_handler, scfcp, 
scfsp->scfs_wait);
                break;
        case SCF_PRIM_ALL:
@@ -348,8 +351,10 @@ static void scftorture_invoke_one(struct scf_statistics 
*scfp, struct torture_ra
                        scfp->n_all_wait++;
                else
                        scfp->n_all++;
-               if (scfcp)
+               if (scfcp) {
+                       barrier(); // Prevent race-reduction compiler 
optimizations.
                        scfcp->scfc_in = true;
+               }
                smp_call_function(scf_handler, scfcp, scfsp->scfs_wait);
                break;
        }
@@ -358,6 +363,7 @@ static void scftorture_invoke_one(struct scf_statistics 
*scfp, struct torture_ra
                        atomic_inc(&n_mb_out_errs); // Leak rather than trash!
                else
                        kfree(scfcp);
+               barrier(); // Prevent race-reduction compiler optimizations.
        }
        if (use_cpus_read_lock)
                cpus_read_unlock();