[tip: locking/urgent] softirq: Avoid bad tracing / lockdep interaction

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     91ea62d58bd661827c328a2c6c02a87fa4aae88b
Gitweb:        https://git.kernel.org/tip/91ea62d58bd661827c328a2c6c02a87fa4aae88b
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Fri, 18 Dec 2020 16:39:14 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 18 Dec 2020 16:53:13 +01:00

softirq: Avoid bad tracing / lockdep interaction

Similar to commit:

  1a63dcd8765b ("softirq: Reorder trace_softirqs_on to prevent lockdep splat")

__local_bh_enable_ip() can also call into tracing with inconsistent
state. Unlike that commit we don't need to bother about the tracepoint
because 'cnt-1' never matches preempt_count() (by construction).

Reported-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201218154519.GW3092@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/softirq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 09229ad..0f1d3a3 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -185,7 +185,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	 * Keep preemption disabled until we are done with
 	 * softirq processing:
 	 */
-	preempt_count_sub(cnt - 1);
+	__preempt_count_sub(cnt - 1);
 
 	if (unlikely(!in_interrupt() && local_softirq_pending())) {
 		/*