[PATCH v3] seccomp: Improve performace by optimizing rmb()

[wanghongzhe]

As Kees haved accepted the v2 patch at a381b70a1 which just
replaced rmb() with smp_rmb(), this patch will base on that and just adjust
the smp_rmb() to the correct position.

As the original comment shown (and indeed it should be):
   /*
    * Make sure that any changes to mode from another thread have
    * been seen after SYSCALL_WORK_SECCOMP was seen.
    */
the smp_rmb() should be put between reading SYSCALL_WORK_SECCOMP and reading
seccomp.mode to make sure that any changes to mode from another thread have
been seen after SYSCALL_WORK_SECCOMP was seen, for TSYNC situation. However,
it is misplaced between reading seccomp.mode and seccomp->filter. This issue
seems to be misintroduced at 13aa72f0fd0a9f98a41cefb662487269e2f1ad65 which
aims to refactor the filter callback and the API. So let's just adjust the
smp_rmb() to the correct position.

A next optimization patch will be provided if this ajustment is appropriate.

v2 -> v3:
 - move the smp_rmb() to the correct position

v1 -> v2:
 - only replace rmb() with smp_rmb()
 - provide the performance test number

RFC -> v1:
 - replace rmb() with smp_rmb()
 - move the smp_rmb() logic to the middle between TIF_SECCOMP and mode

Signed-off-by: wanghongzhe <wanghongzhe@xxxxxxxxxx>
---
 kernel/seccomp.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 1d60fc2c9987..64b236cb8a7f 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1160,12 +1160,6 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 	int data;
 	struct seccomp_data sd_local;
 
-	/*
-	 * Make sure that any changes to mode from another thread have
-	 * been seen after SYSCALL_WORK_SECCOMP was seen.
-	 */
-	smp_rmb();
-
 	if (!sd) {
 		populate_seccomp_data(&sd_local);
 		sd = &sd_local;
@@ -1291,7 +1285,6 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 
 int __secure_computing(const struct seccomp_data *sd)
 {
-	int mode = current->seccomp.mode;
 	int this_syscall;
 
 	if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&
@@ -1301,7 +1294,13 @@ int __secure_computing(const struct seccomp_data *sd)
 	this_syscall = sd ? sd->nr :
 		syscall_get_nr(current, current_pt_regs());
 
-	switch (mode) {
+    /* 
+     * Make sure that any changes to mode from another thread have
+     * been seen after SYSCALL_WORK_SECCOMP was seen.
+     */
+    smp_rmb();
+
+	switch (current->seccomp.mode) {
 	case SECCOMP_MODE_STRICT:
 		__secure_computing_strict(this_syscall);  /* may call do_exit */
 		return 0;
-- 
2.19.1