Commit 00a8478f authored by Uros Bizjak's avatar Uros Bizjak Committed by Steven Rostedt (Google)
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ring_buffer: Use try_cmpxchg instead of cmpxchg 

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
ring_buffer.c. x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

No functional change intended.

Link: https://lore.kernel.org/linux-trace-kernel/20230714154418.8884-1-ubizjak@gmail.com



Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent e7186af7

kernel/trace/ring_buffer.c

+6 −14
Original line number Diff line number Diff line
@@ -692,10 +692,7 @@ static void rb_time_set(rb_time_t *t, u64 val) 
static inline bool

rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set)

{

	unsigned long ret;



	ret = local_cmpxchg(l, expect, set);

	return ret == expect;

	return local_try_cmpxchg(l, &expect, set);

}



static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
@@ -752,9 +749,7 @@ static void rb_time_set(rb_time_t *t, u64 val) 


static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)

{

	u64 val;

	val = local64_cmpxchg(&t->time, expect, set);

	return val == expect;

	return local64_try_cmpxchg(&t->time, &expect, set);

}

#endif
@@ -1494,14 +1489,11 @@ static bool rb_head_page_replace(struct buffer_page *old, 
{

	unsigned long *ptr = (unsigned long *)&old->list.prev->next;

	unsigned long val;

	unsigned long ret;



	val = *ptr & ~RB_FLAG_MASK;

	val |= RB_PAGE_HEAD;



	ret = cmpxchg(ptr, val, (unsigned long)&new->list);



	return ret == val;

	return try_cmpxchg(ptr, &val, (unsigned long)&new->list);

}



/*
@@ -3003,7 +2995,6 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer, 
{

	unsigned long new_index, old_index;

	struct buffer_page *bpage;

	unsigned long index;

	unsigned long addr;

	u64 write_stamp;

	u64 delta;
@@ -3060,8 +3051,9 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer, 
		 */

		old_index += write_mask;

		new_index += write_mask;

		index = local_cmpxchg(&bpage->write, old_index, new_index);

		if (index == old_index) {



		/* caution: old_index gets updated on cmpxchg failure */

		if (local_try_cmpxchg(&bpage->write, &old_index, new_index)) {

			/* update counters */

			local_sub(event_length, &cpu_buffer->entries_bytes);

			return true;