linux-kernel - [PATCH] tracing: zero the pipe cpumask on alloc to avoid spurious -EBUSY

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Message-ID: <20230831125500.986862-1-bfoster@redhat.com>
Date:   Thu, 31 Aug 2023 08:55:00 -0400
From:   Brian Foster <bfoster@...hat.com>
To:     linux-trace-kernel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        zhengyejian1@...wei.com
Subject: [PATCH] tracing: zero the pipe cpumask on alloc to avoid spurious -EBUSY

The pipe cpumask used to serialize opens between the main and percpu
trace pipes is not zeroed or initialized. This can result in
spurious -EBUSY returns if underlying memory is not fully zeroed.
This has been observed by immediate failure to read the main
trace_pipe file on an otherwise newly booted and idle system:

 # cat /sys/kernel/debug/tracing/trace_pipe
 cat: /sys/kernel/debug/tracing/trace_pipe: Device or resource busy

Zero the allocation of pipe_cpumask to avoid the problem.

Fixes: c2489bb7e6be ("tracing: Introduce pipe_cpumask to avoid race on trace_pipes")
Signed-off-by: Brian Foster <bfoster@...hat.com>
---

Hi,

I ran into this problem just recently on one of my test VMs immediately
after updating to a v6.5 base. A revert of the aforementioned commit
addressed the problem. I'm not terribly familiar with the tracing code,
but on further inspection I noticed the cpumask doesn't appear to be
initialized anywhere. I suppose this could alternatively do a
cpumask_clear() or whatever after allocation, but either way this
addresses the problem for me.

Please CC on replies as I'm not subscribed to the list. Thanks.

Brian

 kernel/trace/trace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8e64aaad5361..2656ca3b9b39 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9486,7 +9486,7 @@ static struct trace_array *trace_array_create(const char *name)
 	if (!alloc_cpumask_var(&tr->tracing_cpumask, GFP_KERNEL))
 		goto out_free_tr;

-	if (!alloc_cpumask_var(&tr->pipe_cpumask, GFP_KERNEL))
+	if (!zalloc_cpumask_var(&tr->pipe_cpumask, GFP_KERNEL))
 		goto out_free_tr;

 	tr->trace_flags = global_trace.trace_flags & ~ZEROED_TRACE_FLAGS;
@@ -10431,7 +10431,7 @@ __init static int tracer_alloc_buffers(void)
 	if (trace_create_savedcmd() < 0)
 		goto out_free_temp_buffer;

-	if (!alloc_cpumask_var(&global_trace.pipe_cpumask, GFP_KERNEL))
+	if (!zalloc_cpumask_var(&global_trace.pipe_cpumask, GFP_KERNEL))
 		goto out_free_savedcmd;

 	/* TODO: make the number of buffers hot pluggable with CPUS */
-- 
2.41.0