[PATCH v4 4/8] common: bthread: schedule only in command context

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Originally, I envisioned bthreads as replacement for pollers and
workqueues. But even without preemption, having functions you call
possibly accessing structures you are iterating over can corrupt
memory. This problem exists with pollers as well, but because of their
limited scope, it's harder to shoot your foot with them, as you
don't keep implicit state between poller activations unlike bthreads,
which maintain their stack across context switches.

Limit bthread scope instead to be a replacement for workqueues. This
still allows us to port some classes of state-machine-in-kthread kernel
code, while avoding the aforementioned pitfalls.

Cc: Jan Luebbe <j.luebbe@pengutronix.de>
Cc: Sascha Hauer <a.hauer@pengutronix.de>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 commands/bthread.c | 5 +++++
 common/sched.c     | 6 +++---
 include/slice.h    | 2 --
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/commands/bthread.c b/commands/bthread.c
index 964a1044c58b..0c9c221884e8 100644
--- a/commands/bthread.c
+++ b/commands/bthread.c
@@ -10,12 +10,15 @@
 #include <command.h>
 #include <getopt.h>
 #include <clock.h>
+#include <slice.h>
 
 static int bthread_time(void)
 {
 	uint64_t start = get_time_ns();
 	int i = 0;
 
+	slice_release(&command_slice);
+
 	/*
 	 * How many background tasks can we have in one second?
 	 *
@@ -25,6 +28,8 @@ static int bthread_time(void)
 	while (!is_timeout(start, SECOND))
 		i++;
 
+	slice_acquire(&command_slice);
+
 	return i;
 }
 
diff --git a/common/sched.c b/common/sched.c
index dcf6522ac058..02582b2c5e4d 100644
--- a/common/sched.c
+++ b/common/sched.c
@@ -15,12 +15,12 @@ void resched(void)
 
 	command_slice_acquire();
 
-	if (run_workqueues)
+	if (run_workqueues) {
 		wq_do_all_works();
+		bthread_reschedule();
+	}
 
 	poller_call();
 
 	command_slice_release();
-
-	bthread_reschedule();
 }
diff --git a/include/slice.h b/include/slice.h
index 67d47b9082e6..6c4688e308fa 100644
--- a/include/slice.h
+++ b/include/slice.h
@@ -38,8 +38,6 @@ void command_slice_release(void);
 
 #define assert_command_context() do { \
 	WARN_ONCE(poller_active(), "%s called in poller\n", __func__); \
-	WARN_ONCE(IS_ENABLED(CONFIG_BTHREAD) && !bthread_is_main(current), \
-		  "%s called in secondary bthread\n", __func__); \
 } while (0)
 
 #endif
-- 
2.29.2


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