[RFC][PATCH] ring-buffer: Replace this_cpu_{read,write} with this_cpu_ptr()

[Steven Rostedt]

It has come to my attention that this_cpu_read/write are horrible on
architectures other than x86. Worse yet, they actually disable
preemption or interrupts! This caused some unexpected tracing results
on ARM.

   101.356868: preempt_count_add <-ring_buffer_lock_reserve
   101.356870: preempt_count_sub <-ring_buffer_lock_reserve

The ring_buffer_lock_reserve has recursion protection that requires
accessing a per cpu variable. But since preempt_disable() is traced, it
too got traced while accessing the variable that is suppose to prevent
recursion like this.

The generic version of this_cpu_read() and write() are:

#define _this_cpu_generic_read(pcp)                                     \
({      typeof(pcp) ret__;                                              \
        preempt_disable();                                              \
        ret__ = *this_cpu_ptr(&(pcp));                                  \
        preempt_enable();                                               \
        ret__;                                                          \
})

#define _this_cpu_generic_to_op(pcp, val, op)                           \
do {                                                                    \
        unsigned long flags;                                            \
        raw_local_irq_save(flags);                                      \
        *__this_cpu_ptr(&(pcp)) op val;                                 \
        raw_local_irq_restore(flags);                                   \
} while (0)


Which is unacceptable for locations that know they are within preempt
disabled or interrupt disabled locations.

I may go and remove all this_cpu_read,write() calls from my code
because of this.

Cc: sta...@vger.kernel.org
Cc: Christoph Lameter <c...@linux.com>
Reported-by: Uwe Kleine-König <u.kleine-koe...@pengutronix.de>
Signed-off-by: Steven Rostedt <rost...@goodmis.org>
---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 5040d44fe5a3..be33c6093ca5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2679,7 +2679,11 @@ static DEFINE_PER_CPU(unsigned int, current_context);
 
 static __always_inline int trace_recursive_lock(void)
 {
-       unsigned int val = this_cpu_read(current_context);
+       /*
+        * We can not use this_cpu_read() and this_cpu_write() because
+        * the generic versions call preempt_disable()
+        */
+       unsigned int val = *this_cpu_ptr(&current_context);
        int bit;
 
        if (in_interrupt()) {
@@ -2696,18 +2700,18 @@ static __always_inline int trace_recursive_lock(void)
                return 1;
 
        val |= (1 << bit);
-       this_cpu_write(current_context, val);
+       *this_cpu_ptr(&current_context) = val;
 
        return 0;
 }
 
 static __always_inline void trace_recursive_unlock(void)
 {
-       unsigned int val = this_cpu_read(current_context);
+       unsigned int val = *this_cpu_ptr(&current_context);
 
        val--;
        val &= this_cpu_read(current_context);
-       this_cpu_write(current_context, val);
+       *this_cpu_ptr(&current_context) = val;
 }
 
 #else
--
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