ftrace: Handle tracing when switching between context

BugLink: https://bugs.launchpad.net/bugs/1905612
commit 726b3d3f141fba6f841d715fc4d8a4a84f02c02a upstream.

When an interrupt or NMI comes in and switches the context, there's a delay
from when the preempt_count() shows the update. As the preempt_count() is
used to detect recursion having each context have its own bit get set when
tracing starts, and if that bit is already set, it is considered a recursion
and the function exits. But if this happens in that section where context
has changed but preempt_count() has not been updated, this will be
incorrectly flagged as a recursion.

To handle this case, create another bit call TRANSITION and test it if the
current context bit is already set. Flag the call as a recursion if the
TRANSITION bit is already set, and if not, set it and continue. The
TRANSITION bit will be cleared normally on the return of the function that
set it, or if the current context bit is clear, set it and clear the
TRANSITION bit to allow for another transition between the current context
and an even higher one.

Cc: stable@vger.kernel.org
Fixes: edc15cafcbfa3 ("tracing: Avoid unnecessary multiple recursion checks")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index b84fa0d2d6eae5ef0beb07650cc520ec86018d85..fc3aa81a43e3c8f56abb9f00e6c2482e51742e43 100644 (file)
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -592,6 +592,12 @@ enum {
         * function is called to clear it.
         */
        TRACE_GRAPH_NOTRACE_BIT,
+
+       /*
+        * When transitioning between context, the preempt_count() may
+        * not be correct. Allow for a single recursion to cover this case.
+        */
+       TRACE_TRANSITION_BIT,
 };
 
 #define trace_recursion_set(bit)       do { (current)->trace_recursion |= (1<<(bit)); } while (0)
@@ -646,8 +652,21 @@ static __always_inline int trace_test_and_set_recursion(int start, int max)
                return 0;
 
        bit = trace_get_context_bit() + start;
-       if (unlikely(val & (1 << bit)))
-               return -1;
+       if (unlikely(val & (1 << bit))) {
+               /*
+                * It could be that preempt_count has not been updated during
+                * a switch between contexts. Allow for a single recursion.
+                */
+               bit = TRACE_TRANSITION_BIT;
+               if (trace_recursion_test(bit))
+                       return -1;
+               trace_recursion_set(bit);
+               barrier();
+               return bit + 1;
+       }
+
+       /* Normal check passed, clear the transition to allow it again */
+       trace_recursion_clear(TRACE_TRANSITION_BIT);
 
        val |= 1 << bit;
        current->trace_recursion = val;

diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 69ee8ef12cee372f4f44fca053d2cc5b67182d0a..0838c290ac7f3922987b27e7e0f7a494bd5a787b 100644 (file)
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -492,8 +492,13 @@ trace_selftest_function_recursion(void)
        unregister_ftrace_function(&test_rec_probe);
 
        ret = -1;
-       if (trace_selftest_recursion_cnt != 1) {
-               pr_cont("*callback not called once (%d)* ",
+       /*
+        * Recursion allows for transitions between context,
+        * and may call the callback twice.
+        */
+       if (trace_selftest_recursion_cnt != 1 &&
+           trace_selftest_recursion_cnt != 2) {
+               pr_cont("*callback not called once (or twice) (%d)* ",
                        trace_selftest_recursion_cnt);
                goto out;
        }