AUDIT: Reduce contention in audit_serial()

... by generating serial numbers only if an audit context is actually
_used_, rather than doing so at syscall entry even when the context
isn't necessarily marked auditable.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/kernel/audit.c b/kernel/audit.c
index 27ffcf363f8d..d321e251d32b 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -625,7 +625,9 @@ unsigned int audit_serial(void)
         unsigned int ret;
 
         spin_lock_irqsave(&serial_lock, flags);
-        ret = serial++;
+        do {
+                ret = ++serial;
+        } while (unlikely(!ret));
         spin_unlock_irqrestore(&serial_lock, flags);
 
         return ret;
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 242d45e53738..46b45abceb9a 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -984,7 +984,7 @@ void audit_syscall_entry(struct task_struct *tsk, int arch, int major,
         if (likely(state == AUDIT_DISABLED))
                 return;
 
-        context->serial     = audit_serial();
+        context->serial     = 0;
         context->ctime      = CURRENT_TIME;
         context->in_syscall = 1;
         context->auditable  = !!(state == AUDIT_RECORD_CONTEXT);
@@ -1138,6 +1138,8 @@ void audit_inode(const char *name, const struct inode *inode, unsigned flags)
 void auditsc_get_stamp(struct audit_context *ctx,
                        struct timespec *t, unsigned int *serial)
 {
+        if (!ctx->serial)
+                ctx->serial = audit_serial();
         t->tv_sec  = ctx->ctime.tv_sec;
         t->tv_nsec = ctx->ctime.tv_nsec;
         *serial    = ctx->serial;