Patch "MIPS: SMP: Fix possibility of deadlock when bringing CPUs online" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: SMP: Fix possibility of deadlock when bringing CPUs online

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-smp-fix-possibility-of-deadlock-when-bringing-cpus-online.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Matt Redfearn <[email protected]>
Date: Thu, 22 Sep 2016 17:15:47 +0100
Subject: MIPS: SMP: Fix possibility of deadlock when bringing CPUs online

From: Matt Redfearn <[email protected]>

commit 8f46cca1e6c06a058374816887059bcc017b382f upstream.

This patch fixes the possibility of a deadlock when bringing up secondary CPUs.
The deadlock occurs because the set_cpu_online() is called before synchronise_count_slave(). This can cause a deadlock if the boot CPU, having scheduled another thread, attempts to send an IPI to the secondary CPU, which it sees has been marked online. The secondary is blocked in synchronise_count_slave() waiting for the boot CPU to enter synchronise_count_master(), but the boot cpu is blocked in smp_call_function_many() waiting for the secondary to respond to it's IPI request.

Fix this by marking the CPU online in cpu_callin_map and synchronising counters before declaring the CPU online and calculating the maps for IPIs.

Patchwork: https://patchwork.linux-mips.org/patch/14302/

---
 arch/mips/kernel/smp.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

@@ -320,6 +320,9 @@ asmlinkage void start_secondary(void)
 	cpumask_set_cpu(cpu, &cpu_coherent_mask);
 	notify_cpu_starting(cpu);
 
+	cpumask_set_cpu(cpu, &cpu_callin_map);
+	synchronise_count_slave(cpu);
+
 	set_cpu_online(cpu, true);
 
 	set_cpu_sibling_map(cpu);

@@ -327,10 +330,6 @@ asmlinkage void start_secondary(void)
 
 	calculate_cpu_foreign_map();
 
-	cpumask_set_cpu(cpu, &cpu_callin_map);
-
-	synchronise_count_slave(cpu);
-
 	/*
 	 * irq will be enabled in ->smp_finish(), enabling it too early
 	 * is dangerous.

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/mips-smp-fix-possibility-of-deadlock-when-bringing-cpus-online.patch queue-4.7/mips-paravirt-fix-undefined-reference-to-smp_bootstrap.patch