[RFC PATCH v2 4/9] libcamera: base: log: Make `LogCategory::severity_` atomic

[Barnabás Pőcze]

The severity of a log category may be changed from a different thread,
so it is important to ensure that the reads and writes happen atomically.
Using `std::memory_order_relaxed` should not introduce any synchronization
overhead, it should only guarantee that the operation itself is atomic.

Secondly, inline `LogCategory::setSeverity()`, as it is merely an
assignment, so going through a DSO call is a big pessimization.
`LogCategory` is not part of the public API, so this change has
no external effects.

Thirdly, assert that the atomic variable is lock free so as to ensure
it won't silently fall back to libatomic (or similar) on any platform.
If this assertion fails, this needs to be revisited.

Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/base/log.h | 9 ++++++---
 src/libcamera/base/log.cpp   | 5 +----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/libcamera/base/log.h b/include/libcamera/base/log.h
index 6d2c93019..4137d87a4 100644
--- a/include/libcamera/base/log.h
+++ b/include/libcamera/base/log.h
@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <atomic>
 #include <sstream>
 
 #include <libcamera/base/private.h>
@@ -31,8 +32,8 @@ public:
 	static LogCategory *create(const char *name);
 
 	const std::string &name() const { return name_; }
-	LogSeverity severity() const { return severity_; }
-	void setSeverity(LogSeverity severity);
+	LogSeverity severity() const { return severity_.load(std::memory_order_relaxed); }
+	void setSeverity(LogSeverity severity) { severity_.store(severity, std::memory_order_relaxed); }
 
 	static const LogCategory &defaultCategory();
 
@@ -40,7 +41,9 @@ private:
 	explicit LogCategory(const char *name);
 
 	const std::string name_;
-	LogSeverity severity_;
+
+	std::atomic<LogSeverity> severity_;
+	static_assert(decltype(severity_)::is_always_lock_free);
 };
 
 #define LOG_DECLARE_CATEGORY(name)					\
diff --git a/src/libcamera/base/log.cpp b/src/libcamera/base/log.cpp
index 51a5cd470..2abbad478 100644
--- a/src/libcamera/base/log.cpp
+++ b/src/libcamera/base/log.cpp
@@ -824,15 +824,12 @@ LogCategory::LogCategory(const char *name)
  */
 
 /**
+ * \fn LogCategory::setSeverity(LogSeverity severity)
  * \brief Set the severity of the log category
  *
  * Messages of severity higher than or equal to the severity of the log category
  * are printed, other messages are discarded.
  */
-void LogCategory::setSeverity(LogSeverity severity)
-{
-	severity_ = severity;
-}
 
 /**
  * \brief Retrieve the default log category
-- 
2.48.1