ftrace: Synchronize variable setting with breakpoints

[deliverable/linux.git] / arch / x86 / kernel / ftrace.c

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 32ff36596ab10d65d8d5050402eb8d24a5f3fb6f..2407a6d81cb7de3a8cb23ea718b8b4f979cddbf0 100644 (file)
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -168,7 +168,38 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
        return ret;
 }
 
-int modifying_ftrace_code __read_mostly;
+/*
+ * The modifying_ftrace_code is used to tell the breakpoint
+ * handler to call ftrace_int3_handler(). If it fails to
+ * call this handler for a breakpoint added by ftrace, then
+ * the kernel may crash.
+ *
+ * As atomic_writes on x86 do not need a barrier, we do not
+ * need to add smp_mb()s for this to work. It is also considered
+ * that we can not read the modifying_ftrace_code before
+ * executing the breakpoint. That would be quite remarkable if
+ * it could do that. Here's the flow that is required:
+ *
+ *   CPU-0                          CPU-1
+ *
+ * atomic_inc(mfc);
+ * write int3s
+ *                             <trap-int3> // implicit (r)mb
+ *                             if (atomic_read(mfc))
+ *                                     call ftrace_int3_handler()
+ *
+ * Then when we are finished:
+ *
+ * atomic_dec(mfc);
+ *
+ * If we hit a breakpoint that was not set by ftrace, it does not
+ * matter if ftrace_int3_handler() is called or not. It will
+ * simply be ignored. But it is crucial that a ftrace nop/caller
+ * breakpoint is handled. No other user should ever place a
+ * breakpoint on an ftrace nop/caller location. It must only
+ * be done by this code.
+ */
+atomic_t modifying_ftrace_code __read_mostly;
 
 /*
  * A breakpoint was added to the code address we are about to
@@ -491,11 +522,12 @@ void ftrace_replace_code(int enable)
 
 void arch_ftrace_update_code(int command)
 {
-       modifying_ftrace_code++;
+       /* See comment above by declaration of modifying_ftrace_code */
+       atomic_inc(&modifying_ftrace_code);
 
        ftrace_modify_all_code(command);
 
-       modifying_ftrace_code--;
+       atomic_dec(&modifying_ftrace_code);
 }
 
 int __init ftrace_dyn_arch_init(void *data)