stack.c (_stp_get_uregs): Detect kernel unwind before user unwind case.

Detect and optimize the kernel backtrace() then user backtrace in same
probe case, by reusing the unwind info.

diff --git a/runtime/stack.c b/runtime/stack.c
index fb78781cf85ddd1e9831d4b76fba2b98cb260a11..947e201ddff3bbe69e98e491a97f3dd0b0f2af56 100644 (file)
--- a/runtime/stack.c
+++ b/runtime/stack.c
@@ -166,14 +166,27 @@ static struct pt_regs *_stp_get_uregs(struct context *c)
       else if (c->uregs != NULL && c->kregs != NULL
               && ! (c->probe_flags & _STP_PROBE_STATE_USER_MODE))
        {
-         /* Try to recover the uregs by unwinding from the the kernel
-            probe location. */
          struct unwind_frame_info *info = &c->uwcontext.info;
-         int levels = MAXBACKTRACE;
          int ret = 0;
-         arch_unw_init_frame_info(info, c->kregs, 0);
-         dbug_unwind(1, "Trying to recover... searching for 0x%lx\n",
-                     REG_IP(c->uregs));
+         int levels;
+
+         /* We might be lucky and this probe already ran the kernel
+            unwind to end up in the user regs. */
+         if (UNW_PC(info) == REG_IP(c->uregs))
+           {
+             levels = 0;
+             dbug_unwind(1, "feeling lucky, info pc == uregs pc\n");
+           }
+         else
+           {
+             /* Try to recover the uregs by unwinding from the the kernel
+                probe location. */
+             levels = MAXBACKTRACE;
+             arch_unw_init_frame_info(info, c->kregs, 0);
+             dbug_unwind(1, "Trying to recover... searching for 0x%lx\n",
+                         REG_IP(c->uregs));
+           }
+
          while (levels > 0 && ret == 0 && UNW_PC(info) != REG_IP(c->uregs))
            {
              levels--;
@@ -183,7 +196,7 @@ static struct pt_regs *_stp_get_uregs(struct context *c)
            }
 
          /* Have we arrived where we think user space currently is? */
-         if (ret == 0 && UNW_PC(info) == REG_IP(_stp_current_pt_regs()))
+         if (ret == 0 && UNW_PC(info) == REG_IP(c->uregs))
            {
              /* Note we need to clear this state again when the unwinder
                 has been rerun. See __stp_stack_print invocation below. */