[PATCH 1/1] Fix 'sleeping function called from invalid context' warning in sysrq generated crash.

[Ani Sinha]

Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") 
replaced spin_lock_irqsave() calls with
rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not
disable preemption, faulthandler_disabled() in
__do_page_fault() in x86/fault.c returns false. When the code
later calls might_sleep() in the pagefault handler, we get the
following warning:

BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187
in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a

To fix this, replace RCU call in handle_sysrq() to use SRCU.

Tested this patch on linux 3.18 by booting off one of our boards.

Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")

Signed-off-by: Ani Sinha <ani@xxxxxxxxxx>
---
 drivers/tty/sysrq.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 5381a72..904865f 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -519,10 +519,12 @@ void __handle_sysrq(int key, bool check_mask)
 {
 	struct sysrq_key_op *op_p;
 	int orig_log_level;
-	int i;
+	int i, idx;
+	struct srcu_struct sysrq_rcu;
 
+	init_srcu_struct(&sysrq_rcu);
 	rcu_sysrq_start();
-	rcu_read_lock();
+	idx = srcu_read_lock(&sysrq_rcu);
 	/*
 	 * Raise the apparent loglevel to maximum so that the sysrq header
 	 * is shown to provide the user with positive feedback.  We do not
@@ -564,7 +566,7 @@ void __handle_sysrq(int key, bool check_mask)
 		pr_cont("\n");
 		console_loglevel = orig_log_level;
 	}
-	rcu_read_unlock();
+	srcu_read_unlock(&sysrq_rcu, idx);
 	rcu_sysrq_end();
 }
 
-- 
1.8.1.4

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