trace: Add a generic function to read/write u64 values from tracefs

The hwlat detector and (in preparation for) the osnoise/timerlat tracers
have a set of u64 parameters that the user can read/write via tracefs.
For instance, we have hwlat_detector's window and width.

To reduce the code duplication, hwlat's window and width share the same
read function. However, they do not share the write functions because
they do different parameter checks. For instance, the width needs to
be smaller than the window, while the window needs to be larger
than the window. The same pattern repeats on osnoise/timerlat, and
a large portion of the code was devoted to the write function.

Despite having different checks, the write functions have the same
structure:

   read a user-space buffer
   take the lock that protects the value
   check for minimum and maximum acceptable values
      save the value
   release the lock
   return success or error

To reduce the code duplication also in the write functions, this patch
provides a generic read and write implementation for u64 values that
need to be within some minimum and/or maximum parameters, while
(potentially) being protected by a lock.

To use this interface, the structure trace_min_max_param needs to be
filled:

 struct trace_min_max_param {
         struct mutex    *lock;
         u64             *val;
         u64             *min;
         u64             *max;
 };

The desired value is stored on the variable pointed by *val. If *min
points to a minimum acceptable value, it will be checked during the
write operation. Likewise, if *max points to a maximum allowable value,
it will be checked during the write operation. Finally, if *lock points
to a mutex, it will be taken at the beginning of the operation and
released at the end.

The definition of a trace_min_max_param needs to passed as the
(private) *data for tracefs_create_file(), and the trace_min_max_fops
(added by this patch) as the *fops file_operations.

Link: https://lkml.kernel.org/r/3e35760a7c8b5c55f16ae5ad5fc54a0e71cbe647.1624372313.git.bristot@redhat.com

Cc: Phil Auld <pauld@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Kate Carcia <kcarcia@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Clark Willaims <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index cd80d046c7a5..22f8c652ef8b 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1952,4 +1952,22 @@ static inline bool is_good_name(const char *name)
 	return true;
 }
 
+/*
+ * This is a generic way to read and write a u64 value from a file in tracefs.
+ *
+ * The value is stored on the variable pointed by *val. The value needs
+ * to be at least *min and at most *max. The write is protected by an
+ * existing *lock.
+ */
+struct trace_min_max_param {
+	struct mutex	*lock;
+	u64		*val;
+	u64		*min;
+	u64		*max;
+};
+
+#define U64_STR_SIZE		24	/* 20 digits max */
+
+extern const struct file_operations trace_min_max_fops;
+
 #endif /* _LINUX_KERNEL_TRACE_H */