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 <[email protected]>
---
arch/powerpc/kernel/fadump.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
---
Chanagelog:
v1 -> v3:
- https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/208267.html
v3 -> v4:
- Now the primary CPU (one who triggers dump) waits for all secondary
CPUs to enter and then initiates the crash.
---
diff --git a/arch/powerpc/kernel/fadump.c b/arch/powerpc/kernel/fadump.c
index 59e60a9a9f5c..4953f3246220 100644
--- a/arch/powerpc/kernel/fadump.c
+++ b/arch/powerpc/kernel/fadump.c
@@ -32,6 +32,14 @@
#include <asm/fadump-internal.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 void __init fadump_reserve_crash_area(u64 base);
@@ -46,6 +54,8 @@ struct fadump_mrange_info reserved_mrange_info = { "reserved", NULL, 0, 0, 0 };
#ifdef CONFIG_CMA
static struct cma *fadump_cma;
+static atomic_t cpus_in_crash;
+
/*
* fadump_cma_init() - Initialize CMA area from a fadump reserved memory
*
@@ -596,8 +606,10 @@ early_param("fadump_reserve_mem", early_fadump_reserve_mem);
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;
+ unsigned int ncpus = num_online_cpus() - 1; /* Do not include first CPU */
if (!should_fadump_crash())
return;
@@ -613,6 +625,8 @@ void crash_fadump(struct pt_regs *regs, const char *str)
old_cpu = cmpxchg(&crashing_cpu, -1, this_cpu);
if (old_cpu != -1) {
+ atomic_inc(&cpus_in_crash);
+
/*
* We can't loop here indefinitely. Wait as long as fadump
* is in force. If we race with fadump un-registration this
@@ -636,6 +650,16 @@ void crash_fadump(struct pt_regs *regs, const char *str)
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_crash) < ncpus) && (--msecs > 0))
+ mdelay(1);
+ }
+
fw_dump.ops->fadump_trigger(fdh, str);
}
--
2.25.4
Hi Sourabh,
Thank you for the patch! Yet something to improve:
[auto build test ERROR on powerpc/next]
[also build test ERROR on linus/master linux/master v5.7 next-20200611]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]
url: https://github.com/0day-ci/linux/commits/Sourabh-Jain/powerpc-fadump-fix-race-between-pstore-write-and-fadump-crash-trigger/20200605-033545
base: https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git next
config: powerpc-randconfig-r002-20200612 (attached as .config)
compiler: powerpc64le-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# save the attached .config to linux build tree
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=powerpc
If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <[email protected]>
All errors (new ones prefixed by >>, old ones prefixed by <<):
arch/powerpc/kernel/fadump.c: In function 'crash_fadump':
>> arch/powerpc/kernel/fadump.c:700:15: error: 'cpus_in_crash' undeclared (first use in this function)
700 | atomic_inc(&cpus_in_crash);
| ^~~~~~~~~~~~~
arch/powerpc/kernel/fadump.c:700:15: note: each undeclared identifier is reported only once for each function it appears in
arch/powerpc/kernel/fadump.c: In function 'fadump_update_elfcore_header':
arch/powerpc/kernel/fadump.c:755:17: warning: variable 'elf' set but not used [-Wunused-but-set-variable]
755 | struct elfhdr *elf;
| ^~~
arch/powerpc/kernel/fadump.c: At top level:
arch/powerpc/kernel/fadump.c:1706:22: warning: no previous prototype for 'arch_reserved_kernel_pages' [-Wmissing-prototypes]
1706 | unsigned long __init arch_reserved_kernel_pages(void)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
vim +/cpus_in_crash +700 arch/powerpc/kernel/fadump.c
678
679 void crash_fadump(struct pt_regs *regs, const char *str)
680 {
681 unsigned int msecs;
682 struct fadump_crash_info_header *fdh = NULL;
683 int old_cpu, this_cpu;
684 unsigned int ncpus = num_online_cpus() - 1; /* Do not include first CPU */
685
686 if (!should_fadump_crash())
687 return;
688
689 /*
690 * old_cpu == -1 means this is the first CPU which has come here,
691 * go ahead and trigger fadump.
692 *
693 * old_cpu != -1 means some other CPU has already on it's way
694 * to trigger fadump, just keep looping here.
695 */
696 this_cpu = smp_processor_id();
697 old_cpu = cmpxchg(&crashing_cpu, -1, this_cpu);
698
699 if (old_cpu != -1) {
> 700 atomic_inc(&cpus_in_crash);
701
702 /*
703 * We can't loop here indefinitely. Wait as long as fadump
704 * is in force. If we race with fadump un-registration this
705 * loop will break and then we go down to normal panic path
706 * and reboot. If fadump is in force the first crashing
707 * cpu will definitely trigger fadump.
708 */
709 while (fw_dump.dump_registered)
710 cpu_relax();
711 return;
712 }
713
714 fdh = __va(fw_dump.fadumphdr_addr);
715 fdh->crashing_cpu = crashing_cpu;
716 crash_save_vmcoreinfo();
717
718 if (regs)
719 fdh->regs = *regs;
720 else
721 ppc_save_regs(&fdh->regs);
722
723 fdh->online_mask = *cpu_online_mask;
724
725 /*
726 * If we came in via system reset, wait a while for the secondary
727 * CPUs to enter.
728 */
729 if (TRAP(&(fdh->regs)) == 0x100) {
730 msecs = CRASH_TIMEOUT;
731 while ((atomic_read(&cpus_in_crash) < ncpus) && (--msecs > 0))
732 mdelay(1);
733 }
734
735 fw_dump.ops->fadump_trigger(fdh, str);
736 }
737
---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/[email protected]