1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
|
From e6a23cb5f39c0c3e2cbaef3af7b0ab41fd2104ae Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 30 Nov 2020 01:41:58 +0106
Subject: [PATCH 012/191] printk: use seqcount_latch for clear_seq
kmsg_dump_rewind_nolock() locklessly reads @clear_seq. However,
this is not done atomically. Since @clear_seq is 64-bit, this
cannot be an atomic operation for all platforms. Therefore, use
a seqcount_latch to allow readers to always read a consistent
value.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/printk/printk.c | 58 ++++++++++++++++++++++++++++++++++++------
1 file changed, 50 insertions(+), 8 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8a903faaec4e..b764222b325b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -402,8 +402,21 @@ static u64 console_seq;
static u64 exclusive_console_stop_seq;
static unsigned long console_dropped;
-/* the next printk record to read after the last 'clear' command */
-static u64 clear_seq;
+struct latched_seq {
+ seqcount_latch_t latch;
+ u64 val[2];
+};
+
+/*
+ * The next printk record to read after the last 'clear' command. There are
+ * two copies (updated with seqcount_latch) so that reads can locklessly
+ * access a valid value. Writers are synchronized by @logbuf_lock.
+ */
+static struct latched_seq clear_seq = {
+ .latch = SEQCNT_LATCH_ZERO(clear_seq.latch),
+ .val[0] = 0,
+ .val[1] = 0,
+};
#ifdef CONFIG_PRINTK_CALLER
#define PREFIX_MAX 48
@@ -457,6 +470,31 @@ bool printk_percpu_data_ready(void)
return __printk_percpu_data_ready;
}
+/* Must be called under logbuf_lock. */
+static void latched_seq_write(struct latched_seq *ls, u64 val)
+{
+ raw_write_seqcount_latch(&ls->latch);
+ ls->val[0] = val;
+ raw_write_seqcount_latch(&ls->latch);
+ ls->val[1] = val;
+}
+
+/* Can be called from any context. */
+static u64 latched_seq_read_nolock(struct latched_seq *ls)
+{
+ unsigned int seq;
+ unsigned int idx;
+ u64 val;
+
+ do {
+ seq = raw_read_seqcount_latch(&ls->latch);
+ idx = seq & 0x1;
+ val = ls->val[idx];
+ } while (read_seqcount_latch_retry(&ls->latch, seq));
+
+ return val;
+}
+
/* Return log buffer address */
char *log_buf_addr_get(void)
{
@@ -801,7 +839,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
* like issued by 'dmesg -c'. Reading /dev/kmsg itself
* changes no global state, and does not clear anything.
*/
- user->seq = clear_seq;
+ user->seq = latched_seq_read_nolock(&clear_seq);
break;
case SEEK_END:
/* after the last record */
@@ -960,6 +998,9 @@ void log_buf_vmcoreinfo_setup(void)
VMCOREINFO_SIZE(atomic_long_t);
VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);
+
+ VMCOREINFO_STRUCT_SIZE(latched_seq);
+ VMCOREINFO_OFFSET(latched_seq, val);
}
#endif
@@ -1557,7 +1598,8 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
* Find first record that fits, including all following records,
* into the user-provided buffer for this dump.
*/
- seq = find_first_fitting_seq(clear_seq, -1, size, true, time);
+ seq = find_first_fitting_seq(latched_seq_read_nolock(&clear_seq), -1,
+ size, true, time);
prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX);
@@ -1584,7 +1626,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
}
if (clear)
- clear_seq = seq;
+ latched_seq_write(&clear_seq, seq);
logbuf_unlock_irq();
kfree(text);
@@ -1594,7 +1636,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
static void syslog_clear(void)
{
logbuf_lock_irq();
- clear_seq = prb_next_seq(prb);
+ latched_seq_write(&clear_seq, prb_next_seq(prb));
logbuf_unlock_irq();
}
@@ -3336,7 +3378,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
dumper->active = true;
logbuf_lock_irqsave(flags);
- dumper->cur_seq = clear_seq;
+ dumper->cur_seq = latched_seq_read_nolock(&clear_seq);
dumper->next_seq = prb_next_seq(prb);
logbuf_unlock_irqrestore(flags);
@@ -3534,7 +3576,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
*/
void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
{
- dumper->cur_seq = clear_seq;
+ dumper->cur_seq = latched_seq_read_nolock(&clear_seq);
dumper->next_seq = prb_next_seq(prb);
}
--
2.19.1
|
