The patch titled
Subject: printk: set may_schedule for some of console_trylock() callers
has been added to the -mm tree. Its filename is
printk-set-may_schedule-for-some-of-console_trylock-callers.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/printk-set-may_schedule-for-some-of-console_trylock-callers.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/printk-set-may_schedule-for-some-of-console_trylock-callers.patch
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------------------------------------------------------
From: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Subject: printk: set may_schedule for some of console_trylock() callers
console_unlock() allows to cond_resched() if its caller has set
`console_may_schedule' to 1, since 8d91f8b15361 ("printk: do
cond_resched() between lines while outputting to consoles").
The rules are:
-- console_lock() always sets `console_may_schedule' to 1
-- console_trylock() always sets `console_may_schedule' to 0
However, console_trylock() callers (among them is printk()) do not always
call printk() from atomic contexts, and some of them can cond_resched() in
console_unlock(), so console_trylock() can set `console_may_schedule' to 1
for such processes.
For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock()
always sets `console_may_schedule' to 0.
It's possible to drop explicit preempt_disable()/preempt_enable()
in vprintk_emit(), because console_unlock() and console_trylock()
are now smart enough:
a) console_unlock() does not cond_resched() when it's unsafe
(console_trylock() takes care of that)
b) console_unlock() does can_use_console() check.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Jan Kara <jack@xxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
Cc: Calvin Owens <calvinowens@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/printk/printk.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
diff -puN kernel/printk/printk.c~printk-set-may_schedule-for-some-of-console_trylock-callers kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-set-may_schedule-for-some-of-console_trylock-callers
+++ a/kernel/printk/printk.c
@@ -1769,20 +1769,12 @@ asmlinkage int vprintk_emit(int facility
if (!in_sched) {
lockdep_off();
/*
- * Disable preemption to avoid being preempted while holding
- * console_sem which would prevent anyone from printing to
- * console
- */
- preempt_disable();
-
- /*
* Try to acquire and then immediately release the console
* semaphore. The release will print out buffers and wake up
* /dev/kmsg and syslog() users.
*/
if (console_trylock())
console_unlock();
- preempt_enable();
lockdep_on();
}
@@ -2115,7 +2107,20 @@ int console_trylock(void)
return 0;
}
console_locked = 1;
- console_may_schedule = 0;
+ /*
+ * When PREEMPT_COUNT disabled we can't reliably detect if it's
+ * safe to schedule (e.g. calling printk while holding a spin_lock),
+ * because preempt_disable()/preempt_enable() are just barriers there
+ * and preempt_count() is always 0.
+ *
+ * RCU read sections have a separate preemption counter when
+ * PREEMPT_RCU enabled thus we must take extra care and check
+ * rcu_preempt_depth(), otherwise RCU read sections modify
+ * preempt_count().
+ */
+ console_may_schedule = !oops_in_progress &&
+ preemptible() &&
+ !rcu_preempt_depth();
return 1;
}
EXPORT_SYMBOL(console_trylock);
_
Patches currently in -mm which might be from sergey.senozhatsky@xxxxxxxxx are
mm-workingset-per-cgroup-cache-thrash-detection-fix.patch
zram-export-the-number-of-available-comp-streams.patch
printk-move-can_use_console-out-of-console_trylock_for_printk.patch
printk-set-may_schedule-for-some-of-console_trylock-callers.patch
printk-check-con_enabled-in-have_callable_console.patch
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