From ba608c4fa12cfd0cab0e153249c29441f4dd3312 Mon Sep 17 00:00:00 2001
From: Sourabh Jain <sourabhjain@linux.ibm.com>
Date: Mon, 13 Jul 2020 10:54:35 +0530
Subject: [PATCH] powerpc/fadump: fix race between pstore write and fadump
 crash trigger

References: bsc#1168959 ltc#185010
Patch-mainline: v5.9-rc1
Git-commit: ba608c4fa12cfd0cab0e153249c29441f4dd3312

When we enter into fadump crash path via system reset we fail to update
the pstore.

On the system reset path we first update the pstore then we go for fadump
crash. But the problem here is when all the CPUs try to get the pstore
lock to initiate the pstore write, only one CPUs will acquire the lock
and proceed with the pstore write. Since it in NMI context CPUs that fail
to get lock do not wait for their turn to write to the pstore and simply
proceed with the next operation which is fadump crash. One of the CPU who
proceeded with fadump crash path triggers the crash and does not wait for
the CPU who gets the pstore lock to complete the pstore update.

Timeline diagram to depicts the sequence of events that leads to an
unsuccessful pstore update when we hit fadump crash path via system reset.

                 1    2     3    ...      n   CPU Threads
                 |    |     |             |
                 |    |     |             |
 Reached to   -->|--->|---->| ----------->|
 system reset    |    |     |             |
 path            |    |     |             |
                 |    |     |             |
 Try to       -->|--->|---->|------------>|
 acquire the     |    |     |             |
 pstore lock     |    |     |             |
                 |    |     |             |
                 |    |     |             |
 Got the      -->| +->|     |             |<-+
 pstore lock     | |  |     |             |  |-->  Didn't get the
                 | --------------------------+     lock and moving
                 |    |     |             |        ahead on fadump
                 |    |     |             |        crash path
                 |    |     |             |
  Begins the  -->|    |     |             |
  process to     |    |     |             |<-- Got the chance to
  update the     |    |     |             |    trigger the crash
  pstore         | -> |     |    ... <-   |
                 | |  |     |         |   |
                 | |  |     |         |   |<-- Triggers the
                 | |  |     |         |   |    crash
                 | |  |     |         |   |      ^
                 | |  |     |         |   |      |
  Writing to  -->| |  |     |         |   |      |
  pstore         | |  |     |         |   |      |
                   |                  |          |
       ^           |__________________|          |
       |               CPU Relax                 |
       |                                         |
       +-----------------------------------------+
                          |
                          v
            Race: crash triggered before pstore
                  update completes

To avoid this race condition a barrier is added on crash_fadump path, it
prevents the CPU to trigger the crash until all the online CPUs completes
their task.

A barrier is added to make sure all the secondary CPUs hit the
crash_fadump function before we initiates the crash. A timeout is kept to
ensure the primary CPU (one who initiates the crash) do not wait for
secondary CPUs indefinitely.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200713052435.183750-1-sourabhjain@linux.ibm.com
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
 arch/powerpc/kernel/fadump.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

--- a/arch/powerpc/kernel/fadump.c
+++ b/arch/powerpc/kernel/fadump.c
@@ -44,6 +44,14 @@
 #include <asm/fadump.h>
 #include <asm/setup.h>
 
+/*
+ * The CPU who acquired the lock to trigger the fadump crash should
+ * wait for other CPUs to enter.
+ *
+ * The timeout is in milliseconds.
+ */
+#define CRASH_TIMEOUT		500
+
 static struct fw_dump fw_dump;
 static struct fadump_mem_struct fdm;
 static const struct fadump_mem_struct *fdm_active;
@@ -51,6 +59,8 @@ static const struct fadump_mem_struct *f
 static struct cma *fadump_cma;
 #endif
 
+static atomic_t cpus_in_fadump;
+
 static DEFINE_MUTEX(fadump_mutex);
 struct fadump_mrange_info crash_mrange_info = { "crash", NULL, 0, 0, 0 };
 
@@ -665,8 +675,11 @@ static int register_fw_dump(struct fadum
 
 void crash_fadump(struct pt_regs *regs, const char *str)
 {
+	unsigned int msecs;
 	struct fadump_crash_info_header *fdh = NULL;
 	int old_cpu, this_cpu;
+	/* Do not include first CPU */
+	unsigned int ncpus = num_online_cpus() - 1;
 
 	if (!should_fadump_crash())
 		return;
@@ -682,6 +695,8 @@ void crash_fadump(struct pt_regs *regs,
 	old_cpu = cmpxchg(&crashing_cpu, -1, this_cpu);
 
 	if (old_cpu != -1) {
+		atomic_inc(&cpus_in_fadump);
+
 		/*
 		 * We can't loop here indefinitely. Wait as long as fadump
 		 * is in force. If we race with fadump un-registration this
@@ -705,6 +720,16 @@ void crash_fadump(struct pt_regs *regs,
 
 	fdh->online_mask = *cpu_online_mask;
 
+	/*
+	 * If we came in via system reset, wait a while for the secondary
+	 * CPUs to enter.
+	 */
+	if (TRAP(&(fdh->regs)) == 0x100) {
+		msecs = CRASH_TIMEOUT;
+		while ((atomic_read(&cpus_in_fadump) < ncpus) && (--msecs > 0))
+			mdelay(1);
+	}
+
 	/* Call ibm,os-term rtas call to trigger firmware assisted dump */
 	rtas_os_term((char *)str);
 }
