[PATCH 4/4] 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>
---
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 | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 0f27af6ade3e..3d12ce8a591e 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -2466,16 +2466,14 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 	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();
 
@@ -2524,7 +2522,6 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 	 * Setup is done.
 	 */
 	mutex_unlock(&osn_var->lock);
-	mutex_unlock(&interface_lock);
 
 	tlat = this_cpu_tmr_var();
 	tlat->count = 0;
@@ -2536,7 +2533,6 @@ static int timerlat_fd_open(struct inode *inode, struct file *file)
 
 err:
 	mutex_unlock(&osn_var->lock);
-	mutex_unlock(&interface_lock);
 	migrate_enable();
 	return ret;
 };
-- 
2.54.0