[PATCH v2 6/9] tracing/osnoise: Take trace_types_lock in timerlat_fd_open

[Crystal Wood <crwood@redhat.com>]

This prevents a user fd from being opened when osnoise_options_write()
has already stopped threads but not yet set OSN_WORKLOAD, and matches what
all callers of start_kthread() do.

The original intent was probably for interface_lock to cover things like
this, but it never covered the full race window, and currently can't
due to lock order issues stemming from using interface_lock in
osnoise_sleep().  There does not appear to be any remaining reason
to take interface_lock in timerlat_fd_open(), so remove it.

Signed-off-by: Crystal Wood <crwood@redhat.com>
---
v2: convert rest of function to guards

Do we really need that interface_lock in osnoise_sleep()?  Technically
we should probably be grabbing it every time we access osnoise_data, but
we don't seem to do so anywhere else.  Perhaps this was due to concerns
about the atomicity of "period - runtime", but that's just synchronization
theater given that userspace has no way of writing to both atomically.

In any case, we shouldn't use the same lock for protecting osnoise_data
members that we use for major reconfiguration procedures.
---
 kernel/trace/trace_osnoise.c | 43 +++++++++---------------------------
 1 file changed, 10 insertions(+), 33 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 92b1216d5740..eaecd56492e3 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -2497,28 +2497,23 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 	struct osnoise_variables *osn_var;
 	struct timerlat_variables *tlat;
 	long cpu = (long) inode->i_cdev;
-	int ret = 0;
 
-	mutex_lock(&interface_lock);
+	guard(mutex)(&trace_types_lock);
 
 	/*
 	 * This file is accessible only if timerlat is enabled, and
 	 * NO_OSNOISE_WORKLOAD is set.
 	 */
-	if (!timerlat_enabled() || test_bit(OSN_WORKLOAD, &osnoise_options)) {
-		mutex_unlock(&interface_lock);
+	if (!timerlat_enabled() || test_bit(OSN_WORKLOAD, &osnoise_options))
 		return -EINVAL;
-	}
 
-	migrate_disable();
+	guard(migrate)();
 
 	osn_var = this_cpu_osn_var();
-	mutex_lock(&osn_var->lock);
+	guard(mutex)(&osn_var->lock);
 
-	if (osn_var->fdd) {
-		ret = -EBUSY;
-		goto err;
-	}
+	if (osn_var->fdd)
+		return -EBUSY;
 
 	WARN_ON_ONCE(osn_var->kthread);
 
@@ -2529,16 +2524,12 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 	 * worth doing this basic acceptance test to avoid obviusly wrong
 	 * setup.
 	 */
-	if (current->nr_cpus_allowed > 1 ||  cpu != smp_processor_id()) {
-		ret = -EPERM;
-		goto err;
-	}
+	if (current->nr_cpus_allowed > 1 ||  cpu != smp_processor_id())
+		return -EPERM;
 
 	fdd = kmalloc(sizeof(*fdd), GFP_KERNEL);
-	if (!fdd) {
-		ret = -ENOMEM;
-		goto err;
-	}
+	if (!fdd)
+		return -ENOMEM;
 
 	/*
 	 * From now on, it is good to go.
@@ -2553,25 +2544,11 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 	osn_var->pid = current->pid;
 	osn_var->fdd = fdd;
 
-	/*
-	 * Setup is done.
-	 */
-	mutex_unlock(&osn_var->lock);
-	mutex_unlock(&interface_lock);
-
 	tlat = this_cpu_tmr_var();
 	tlat->count = 0;
 
 	hrtimer_setup(&tlat->timer, timerlat_irq, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED_HARD);
-
-	migrate_enable();
 	return 0;
-
-err:
-	mutex_unlock(&osn_var->lock);
-	mutex_unlock(&interface_lock);
-	migrate_enable();
-	return ret;
 };
 
 /*
-- 
2.54.0