diff options
Diffstat (limited to 'arch/x86/kernel/ftrace.c')
| -rw-r--r-- | arch/x86/kernel/ftrace.c | 38 |
1 files changed, 35 insertions, 3 deletions
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c index 32ff36596ab1..2407a6d81cb7 100644 --- 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) |