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Subject[PATCH 2/8] printk: Release lockbuf_lock before calling console_trylock_for_printk()
Date
There's no reason to hold lockbuf_lock when entering
console_trylock_for_printk(). The first thing this function does is
calling down_trylock(console_sem) and if that fails it immediately
unlocks lockbuf_lock. So lockbuf_lock isn't needed for that branch.
When down_trylock() succeeds, the rest of console_trylock() is OK
without lockbuf_lock (it is called without it from other places), and
the only remaining thing in console_trylock_for_printk() is
can_use_console() call. For that call console_sem is enough (it
iterates all consoles and checks CON_ANYTIME flag).

So we drop logbuf_lock before entering console_trylock_for_printk()
which simplifies the code.

Signed-off-by: Jan Kara <jack@suse.cz>
---
kernel/printk/printk.c | 49 +++++++++++++++++--------------------------------
1 file changed, 17 insertions(+), 32 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bd7ee2a9f960..7a8ffd89875c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -249,9 +249,6 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;

-/* cpu currently holding logbuf_lock */
-static volatile unsigned int logbuf_cpu = UINT_MAX;
-
/* human readable text of the record */
static char *log_text(const struct printk_log *msg)
{
@@ -1332,36 +1329,22 @@ static inline int can_use_console(unsigned int cpu)
* messages from a 'printk'. Return true (and with the
* console_lock held, and 'console_locked' set) if it
* is successful, false otherwise.
- *
- * This gets called with the 'logbuf_lock' spinlock held and
- * interrupts disabled. It should return with 'lockbuf_lock'
- * released but interrupts still disabled.
*/
static int console_trylock_for_printk(unsigned int cpu)
- __releases(&logbuf_lock)
{
- int retval = 0, wake = 0;
-
- if (console_trylock()) {
- retval = 1;
-
- /*
- * If we can't use the console, we need to release
- * the console semaphore by hand to avoid flushing
- * the buffer. We need to hold the console semaphore
- * in order to do this test safely.
- */
- if (!can_use_console(cpu)) {
- console_locked = 0;
- wake = 1;
- retval = 0;
- }
- }
- logbuf_cpu = UINT_MAX;
- raw_spin_unlock(&logbuf_lock);
- if (wake)
+ if (!console_trylock())
+ return 0;
+ /*
+ * If we can't use the console, we need to release the console
+ * semaphore by hand to avoid flushing the buffer. We need to hold the
+ * console semaphore in order to do this test safely.
+ */
+ if (!can_use_console(cpu)) {
+ console_locked = 0;
up(&console_sem);
- return retval;
+ return 0;
+ }
+ return 1;
}

int printk_delay_msec __read_mostly;
@@ -1494,6 +1477,9 @@ asmlinkage int vprintk_emit(int facility, int level,
unsigned long flags;
int this_cpu;
int printed_len = 0;
+ /* cpu currently holding logbuf_lock in this function */
+ static volatile unsigned int logbuf_cpu = UINT_MAX;
+

boot_delay_msec(level);
printk_delay();
@@ -1609,13 +1595,12 @@ asmlinkage int vprintk_emit(int facility, int level,
}
printed_len += text_len;

+ logbuf_cpu = UINT_MAX;
+ raw_spin_unlock(&logbuf_lock);
/*
* Try to acquire and then immediately release the console semaphore.
* The release will print out buffers and wake up /dev/kmsg and syslog()
* users.
- *
- * The console_trylock_for_printk() function will release 'logbuf_lock'
- * regardless of whether it actually gets the console semaphore or not.
*/
if (console_trylock_for_printk(this_cpu))
console_unlock();
--
1.8.1.4


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