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From:   "Guilherme G. Piccoli" <gpiccoli@igalia.com>
To:     linux-kernel@vger.kernel.org, rostedt@goodmis.org
Cc:     rcu@vger.kernel.org, kernel-dev@igalia.com, kernel@gpiccoli.net,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: [PATCH] ftrace: Prevent RCU stall on PREEMPT_VOLUNTARY kernels
Date:   Tue, 15 Nov 2022 17:48:47 -0300
Message-Id: <20221115204847.593616-1-gpiccoli@igalia.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The function match_records() may take a while due to a large
number of string comparisons, so when in PREEMPT_VOLUNTARY
kernels we could face RCU stalls due to that.

Add a cond_resched() to prevent that.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org> # from RCU CPU stall warning perspective
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---

Hi Steve / Paul, thanks for the discussions on the first thread [0],
much appreciated! Here is the "official" version.

Steve: lemme know if it's good for you, and in case you prefer to
send it yourself (since you proposed it on IRC), fine by me!

Paul: kept your ACK (thanks for that BTW) even though I changed the
place of cond_resched() to align with Steve's preference. Lemme know
in case you want to drop this ACK.

Cheers,

Guilherme


[0] https://lore.kernel.org/lkml/1ef5fe19-a82f-835e-fda5-455e9c2b94b4@igalia.com/


 kernel/trace/ftrace.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 7dc023641bf1..80639bdb85f6 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4192,6 +4192,7 @@ match_records(struct ftrace_hash *hash, char *func, int len, char *mod)
 			}
 			found = 1;
 		}
+		cond_resched();
 	} while_for_each_ftrace_rec();
  out_unlock:
 	mutex_unlock(&ftrace_lock);
-- 
2.38.0