printk: Fix console_sem vs logbuf_lock unlock race (4f2a8d3c) · Commits · 戴 / test

Fix up the fallout from commit ("printk: Release console_sem after logbuf_lock"). The reason for unlocking the console_sem under the logbuf_lock is that a concurrent printk() might fill up the buffer but fail to acquire the console sem, resulting in a missed write to the console until a subsequent console_sem acquire/release cycle. Signed-off-by:

Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: efault@gmx.de Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1308734409.1022.14.camel@twins Signed-off-by:

Ingo Molnar <mingo@elte.hu>

kernel/printk.c

+18 −2

Original line number	Diff line number	Diff line
		@@ -1244,7 +1244,7 @@ void console_unlock(void)
		{
		unsigned long flags;
		unsigned _con_start, _log_end;
		unsigned wake_klogd = 0;
		unsigned wake_klogd = 0, retry = 0;

		if (console_suspended) {
		up(&console_sem);
		@@ -1253,6 +1253,7 @@ void console_unlock(void)

		console_may_schedule = 0;

		again:
		for ( ; ; ) {
		spin_lock_irqsave(&logbuf_lock, flags);
		wake_klogd \|= log_start - log_end;
		@@ -1273,8 +1274,23 @@ void console_unlock(void)
		if (unlikely(exclusive_console))
		exclusive_console = NULL;

		spin_unlock_irqrestore(&logbuf_lock, flags);
		spin_unlock(&logbuf_lock);

		up(&console_sem);

		/*
		* Someone could have filled up the buffer again, so re-check if there's
		* something to flush. In case we cannot trylock the console_sem again,
		* there's a new owner and the console_unlock() from them will do the
		* flush, no worries.
		*/
		spin_lock(&logbuf_lock);
		if (con_start != log_end)
		retry = 1;
		spin_unlock_irqrestore(&logbuf_lock, flags);
		if (retry && console_trylock())
		goto again;

		if (wake_klogd)
		wake_up_klogd();
		}

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