110 lines
4.4 KiB
Diff
110 lines
4.4 KiB
Diff
From a7014f6d411685a5691be0f59c0634e165771811 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Jan=20Pokorn=C3=BD?= <jpokorny@redhat.com>
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Date: Fri, 6 Oct 2017 17:17:26 +0200
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Subject: [PATCH 1/6] Med: qblog.h: better explanation + behaviour of
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QB_LOG_INIT_DATA
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Based on better understanding how link-time callsite collection works,
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put a better description for the macro. Also based on poor user
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experience in case that feature does not work well, say because
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the linker deliberately changes the previously settled visibility
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of the section boundary symbols (happened in ld from binutils-2.29,
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fix is forthcoming), tweak the assertion message a bit, together
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with an extension of the general intro to point that macro out.
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Also play fair, include the <assert.h> header, which this macro
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requires.
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- use case:
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/usr/sbin/pacemakerd --features
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- before:
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pacemakerd: utils.c:69: common:
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Assertion `0' failed
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- after:
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pacemakerd: utils.c:69: common:
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Assertion `"non-empty callsite section" && QB_ATTR_SECTION_START != QB_ATTR_SECTION_STOP' failed.
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Inspired by the suggestion of Ferenc Wágner (for the subsequent
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commit, actually).
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And regarding:
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> as a side effect, it can ensure the boundary-denoting symbols for
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> the target collection area are kept alive with some otherwise unkind
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> linkers
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this was actually empirically discovered in one particular combination
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with ld.bfd @ binutils 2.29, and extensively with 2.29.1 (placing here
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a forward reference for the following commits that elaborate on the
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libqb-target cross-combination matrix and the findings).
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Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
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---
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include/qb/qblog.h | 34 +++++++++++++++++++++++++++++-----
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1 file changed, 29 insertions(+), 5 deletions(-)
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diff --git a/include/qb/qblog.h b/include/qb/qblog.h
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index 3cb4eef..d39803f 100644
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--- a/include/qb/qblog.h
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+++ b/include/qb/qblog.h
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@@ -42,6 +42,10 @@ extern "C" {
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#undef QB_HAVE_ATTRIBUTE_SECTION
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#endif /* S_SPLINT_S */
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+#ifdef QB_HAVE_ATTRIBUTE_SECTION
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+#include <assert.h> /* possibly needed for QB_LOG_INIT_DATA */
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+#endif
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+
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/**
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* @file qblog.h
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* The logging API provides four main parts (basics, filtering, threading & blackbox).
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@@ -64,6 +68,17 @@ extern "C" {
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* }
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* @endcode
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*
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+ * @note
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+ * In practice, such a minimalistic approach hardly caters real use cases.
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+ * Following section discusses the customization. Moreover, it's quite
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+ * vital to instrument the target user of this logging subsystem with
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+ * @c QB_LOG_INIT_DATA() macro placed in the top file scope in an exactly
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+ * one source file (preferably the main one) to be compiled into the
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+ * resulting binary. This is a self-defensive measure for when the
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+ * linker-assisted collection of callsite data silently fails, which
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+ * could otherwise go unnoticed, causing troubles down the road.
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+ *
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+ *
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* @par Configuring log targets.
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* A log target can be syslog, stderr, the blackbox, stdout, or a text file.
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* By default only syslog is enabled.
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@@ -268,11 +283,20 @@ typedef void (*qb_log_filter_fn)(struct qb_log_callsite * cs);
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extern struct qb_log_callsite QB_ATTR_SECTION_START[];
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extern struct qb_log_callsite QB_ATTR_SECTION_STOP[];
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-/* mere linker sanity check, possible future extension for internal purposes */
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-#define QB_LOG_INIT_DATA(name) \
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- void name(void); \
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- void name(void) \
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- { if (QB_ATTR_SECTION_START == QB_ATTR_SECTION_STOP) assert(0); } \
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+/* optional on-demand self-check of (1) toolchain sanity (prerequisite for
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+ * the logging subsystem to work properly) and (2) non-void active use of
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+ * logging (satisfied with a justifying existence of a logging callsite as
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+ * defined with a @c qb_logt invocation) at the target binary, which is
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+ * presumably assured by it's author as of relying on this very macro;
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+ * only effective when link-time ("run-time amortizing") callsite collection
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+ * is; as a side effect, it can ensure the boundary-denoting symbols for the
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+ * target collection area are kept alive with some otherwise unkind linkers;
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+ * may be extended in future for more in-depth self-validation */
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+#define QB_LOG_INIT_DATA(name) \
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+ void name(void); \
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+ void name(void) \
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+ { assert("non-empty callsite section" \
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+ && QB_ATTR_SECTION_START != QB_ATTR_SECTION_STOP); } \
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void __attribute__ ((constructor)) name(void);
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#else
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#define QB_LOG_INIT_DATA(name)
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--
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2.14.2
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