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Subject[V4 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
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Currently, panic() and crash_kexec() can be called at the same time.
For example (x86 case):

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus

CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump
fails.

In another case:

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
<NMI>
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude
others by using atomic_t panic_cpu.

V4:
- Use new __crash_kexec(), no exclusion check version of crash_kexec(),
instead of checking if panic_cpu is the current cpu or not

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock
to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
---
include/linux/kexec.h | 1 +
kernel/kexec_core.c | 26 +++++++++++++++++++++++++-
kernel/panic.c | 4 ++--
3 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index d140b1e..f0cd2fa 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -237,6 +237,7 @@ extern int kexec_purgatory_get_set_symbol(struct kimage *image,
unsigned int size, bool get_value);
extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
const char *name);
+extern void __crash_kexec(struct pt_regs *);
extern void crash_kexec(struct pt_regs *);
int kexec_should_crash(struct task_struct *);
void crash_save_cpu(struct pt_regs *regs, int cpu);
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 201b453..4edb20a 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -853,7 +853,8 @@ int kimage_load_segment(struct kimage *image,
struct kimage *kexec_crash_image;
int kexec_load_disabled;

-void crash_kexec(struct pt_regs *regs)
+/* No panic_cpu check version of crash_kexec */
+void __crash_kexec(struct pt_regs *regs)
{
/* Take the kexec_mutex here to prevent sys_kexec_load
* running on one cpu from replacing the crash kernel
@@ -876,6 +877,29 @@ void crash_kexec(struct pt_regs *regs)
}
}

+void crash_kexec(struct pt_regs *regs)
+{
+ int old_cpu, this_cpu;
+
+ /*
+ * Only one CPU is allowed to execute the crash_kexec() code as with
+ * panic(). Otherwise parallel calls of panic() and crash_kexec()
+ * may stop each other. To exclude them, we use panic_cpu here too.
+ */
+ this_cpu = raw_smp_processor_id();
+ old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+ if (old_cpu == -1) {
+ /* This is the 1st CPU which comes here, so go ahead. */
+ __crash_kexec(regs);
+
+ /*
+ * Reset panic_cpu to allow another panic()/crash_kexec()
+ * call.
+ */
+ atomic_xchg(&panic_cpu, -1);
+ }
+}
+
size_t crash_get_memory_size(void)
{
size_t size = 0;
diff --git a/kernel/panic.c b/kernel/panic.c
index cddbfe0..994be45 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -137,7 +137,7 @@ void panic(const char *fmt, ...)
* the "crash_kexec_post_notifiers" option to the kernel.
*/
if (!crash_kexec_post_notifiers)
- crash_kexec(NULL);
+ __crash_kexec(NULL);

/*
* Note smp_send_stop is the usual smp shutdown function, which
@@ -162,7 +162,7 @@ void panic(const char *fmt, ...)
* more unstable, it can increase risks of the kdump failure too.
*/
if (crash_kexec_post_notifiers)
- crash_kexec(NULL);
+ __crash_kexec(NULL);

bust_spinlocks(0);




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