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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org
Subject: [PATCH 4.14 79/89] Revert "printk: Never set console_may_schedule in console_trylock()"
Date:   Fri, 14 Dec 2018 13:00:32 +0100
Message-Id: <20181214115733.594242985@linuxfoundation.org>
In-Reply-To: <20181214115729.658859279@linuxfoundation.org>
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4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

This reverts commit c9b8d580b3fb0ab65d37c372aef19a318fda3199.

This is just a technical revert to make the printk fix apply cleanly,
this patch will be re-picked in about 3 commits.
---
 kernel/printk/printk.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index a9cf2e15f6a3..7161312593dd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1762,12 +1762,6 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	/* If called from the scheduler, we can not call up(). */
 	if (!in_sched) {
-		/*
-		 * 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
@@ -1775,7 +1769,6 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 */
 		if (console_trylock())
 			console_unlock();
-		preempt_enable();
 	}
 
 	return printed_len;
@@ -2090,7 +2083,20 @@ 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);
-- 
2.19.1