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2023-12-08 01573e231f18eb2d99162747186f59511f56b64d

 kernel/kernel/trace/trace_kdb.c
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@@ -17,22 +17,16 @@
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 #include "trace.h"
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 #include "trace_output.h"
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-static void ftrace_dump_buf(int skip_lines, long cpu_file)
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+static struct trace_iterator iter;
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+static struct ring_buffer_iter *buffer_iter[CONFIG_NR_CPUS];
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+
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+static void ftrace_dump_buf(int skip_entries, long cpu_file)
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 {
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-	/* use static because iter can be a bit big for the stack */
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-	static struct trace_iterator iter;
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-	static struct ring_buffer_iter *buffer_iter[CONFIG_NR_CPUS];
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 	struct trace_array *tr;
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 	unsigned int old_userobj;
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 	int cnt = 0, cpu;
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-	trace_init_global_iter(&iter);
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-	iter.buffer_iter = buffer_iter;
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 	tr = iter.tr;
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-
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-	for_each_tracing_cpu(cpu) {
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-		atomic_inc(&per_cpu_ptr(iter.trace_buffer->data, cpu)->disabled);
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-	}
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 	old_userobj = tr->trace_flags;
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@@ -40,6 +34,8 @@
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 	tr->trace_flags &= ~TRACE_ITER_SYM_USEROBJ;
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 	kdb_printf("Dumping ftrace buffer:\n");
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+	if (skip_entries)
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+		kdb_printf("(skipping %d entries)\n", skip_entries);
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 	trace_iterator_reset(&iter);
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 	iter.iter_flags |= TRACE_FILE_LAT_FMT;
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@@ -47,7 +43,7 @@
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 	if (cpu_file == RING_BUFFER_ALL_CPUS) {
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 		for_each_tracing_cpu(cpu) {
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 			iter.buffer_iter[cpu] =
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-			ring_buffer_read_prepare(iter.trace_buffer->buffer,
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+			ring_buffer_read_prepare(iter.array_buffer->buffer,
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 						 cpu, GFP_ATOMIC);
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 			ring_buffer_read_start(iter.buffer_iter[cpu]);
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 			tracing_iter_reset(&iter, cpu);
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@@ -55,7 +51,7 @@
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 	} else {
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 		iter.cpu_file = cpu_file;
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 		iter.buffer_iter[cpu_file] =
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-			ring_buffer_read_prepare(iter.trace_buffer->buffer,
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+			ring_buffer_read_prepare(iter.array_buffer->buffer,
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 						 cpu_file, GFP_ATOMIC);
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 		ring_buffer_read_start(iter.buffer_iter[cpu_file]);
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 		tracing_iter_reset(&iter, cpu_file);
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@@ -66,11 +62,11 @@
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 			kdb_printf("---------------------------------\n");
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 		cnt++;
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64
 
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-		if (!skip_lines) {
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+		if (!skip_entries) {
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 			print_trace_line(&iter);
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 			trace_printk_seq(&iter.seq);
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 		} else {
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-			skip_lines--;
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+			skip_entries--;
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 		}
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 		if (KDB_FLAG(CMD_INTERRUPT))
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@@ -86,10 +82,6 @@
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 	tr->trace_flags = old_userobj;
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83
 
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 	for_each_tracing_cpu(cpu) {
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-		atomic_dec(&per_cpu_ptr(iter.trace_buffer->data, cpu)->disabled);
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-	}
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-
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-	for_each_tracing_cpu(cpu) {
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 		if (iter.buffer_iter[cpu]) {
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 			ring_buffer_read_finish(iter.buffer_iter[cpu]);
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 			iter.buffer_iter[cpu] = NULL;
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@@ -102,17 +94,19 @@
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  */
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 static int kdb_ftdump(int argc, const char **argv)
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 {
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-	int skip_lines = 0;
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+	int skip_entries = 0;
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 	long cpu_file;
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 	char *cp;
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+	int cnt;
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+	int cpu;
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 	if (argc > 2)
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 		return KDB_ARGCOUNT;
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 	if (argc) {
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-		skip_lines = simple_strtol(argv[1], &cp, 0);
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+		skip_entries = simple_strtol(argv[1], &cp, 0);
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 		if (*cp)
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-			skip_lines = 0;
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+			skip_entries = 0;
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 	}
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 	if (argc == 2) {
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@@ -125,7 +119,29 @@
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 	}
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 	kdb_trap_printk++;
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-	ftrace_dump_buf(skip_lines, cpu_file);
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+
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+	trace_init_global_iter(&iter);
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+	iter.buffer_iter = buffer_iter;
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+
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+	for_each_tracing_cpu(cpu) {
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+		atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
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+	}
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+
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+	/* A negative skip_entries means skip all but the last entries */
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+	if (skip_entries < 0) {
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+		if (cpu_file == RING_BUFFER_ALL_CPUS)
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+			cnt = trace_total_entries(NULL);
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+		else
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+			cnt = trace_total_entries_cpu(NULL, cpu_file);
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+		skip_entries = max(cnt + skip_entries, 0);
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+	}
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+
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+	ftrace_dump_buf(skip_entries, cpu_file);
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+
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+	for_each_tracing_cpu(cpu) {
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+		atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
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+	}
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+
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 	kdb_trap_printk--;
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 	return 0;
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@@ -133,8 +149,9 @@
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 static __init int kdb_ftrace_register(void)
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 {
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-	kdb_register_flags("ftdump", kdb_ftdump, "[skip_#lines] [cpu]",
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-			    "Dump ftrace log", 0, KDB_ENABLE_ALWAYS_SAFE);
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+	kdb_register_flags("ftdump", kdb_ftdump, "[skip_#entries] [cpu]",
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+			    "Dump ftrace log; -skip dumps last #entries", 0,
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+			    KDB_ENABLE_ALWAYS_SAFE);
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 	return 0;
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 }
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