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authorPetr Mladek <pmladek@suse.com>2018-02-26 15:44:20 +0100
committerPetr Mladek <pmladek@suse.com>2018-02-27 10:25:50 +0100
commitc14376de3a1befa70d9811ca2872d47367b48767 (patch)
treedb46c75232ae3777fb20438f4fb0415ade0e1e5a /kernel/printk
parentvsprintf: avoid misleading "(null)" for %px (diff)
downloadlinux-c14376de3a1befa70d9811ca2872d47367b48767.tar.xz
linux-c14376de3a1befa70d9811ca2872d47367b48767.zip
printk: Wake klogd when passing console_lock owner
wake_klogd is a local variable in console_unlock(). The information is lost when the console_lock owner using the busy wait added by the commit dbdda842fe96f8932 ("printk: Add console owner and waiter logic to load balance console writes"). The following race is possible: CPU0 CPU1 console_unlock() for (;;) /* calling console for last message */ printk() log_store() log_next_seq++; /* see new message */ if (seen_seq != log_next_seq) { wake_klogd = true; seen_seq = log_next_seq; } console_lock_spinning_enable(); if (console_trylock_spinning()) /* spinning */ if (console_lock_spinning_disable_and_check()) { printk_safe_exit_irqrestore(flags); return; console_unlock() if (seen_seq != log_next_seq) { /* already seen */ /* nothing to do */ Result: Nobody would wakeup klogd. One solution would be to make a global variable from wake_klogd. But then we would need to manipulate it under a lock or so. This patch wakes klogd also when console_lock is passed to the spinning waiter. It looks like the right way to go. Also userspace should have a chance to see and store any "flood" of messages. Note that the very late klogd wake up was a historic solution. It made sense on single CPU systems or when sys_syslog() operations were synchronized using the big kernel lock like in v2.1.113. But it is questionable these days. Fixes: dbdda842fe96f8932 ("printk: Add console owner and waiter logic to load balance console writes") Link: http://lkml.kernel.org/r/20180226155734.dzwg3aovqnwtvkoy@pathway.suse.cz Cc: Steven Rostedt <rostedt@goodmis.org> Cc: linux-kernel@vger.kernel.org Cc: Tejun Heo <tj@kernel.org> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index db4b9b8929eb..4d818642ac0e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2397,7 +2397,7 @@ skip:
if (console_lock_spinning_disable_and_check()) {
printk_safe_exit_irqrestore(flags);
- return;
+ goto out;
}
printk_safe_exit_irqrestore(flags);
@@ -2430,6 +2430,7 @@ skip:
if (retry && console_trylock())
goto again;
+out:
if (wake_klogd)
wake_up_klogd();
}