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Date: Wed, 9 Sep 2020 07:46:07 +0200
From: Joerg Vehlow <lkml@...coder.de>
To: peterz@...radead.org
Cc: Steven Rostedt <rostedt@...dmis.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Huang Ying <ying.huang@...el.com>,
linux-kernel@...r.kernel.org,
Joerg Vehlow <joerg.vehlow@...-tech.de>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in interrupt
context
Hi,
here is the new version of the patch based on Peters suggestion
It looks like it works fine. I added the BUG_ON to __crash_kexec,
because it is a precondition, that panic_cpu is set correctly, otherwise
the whole locking logic fails.
The mutex_trylock can still be used, because it is only in syscall
context and no interrupt context.
Jörg
---
kernel/kexec.c | 8 ++--
kernel/kexec_core.c | 86 +++++++++++++++++++++++++++--------------
kernel/kexec_file.c | 4 +-
kernel/kexec_internal.h | 6 ++-
4 files changed, 69 insertions(+), 35 deletions(-)
diff --git a/kernel/kexec.c b/kernel/kexec.c
index f977786fe498..118a012aeac2 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -255,12 +255,12 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry,
unsigned long, nr_segments,
*
* KISS: always take the mutex.
*/
- if (!mutex_trylock(&kexec_mutex))
+ if (!kexec_trylock())
return -EBUSY;
result = do_kexec_load(entry, nr_segments, segments, flags);
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return result;
}
@@ -309,12 +309,12 @@ COMPAT_SYSCALL_DEFINE4(kexec_load, compat_ulong_t,
entry,
*
* KISS: always take the mutex.
*/
- if (!mutex_trylock(&kexec_mutex))
+ if (!kexec_trylock())
return -EBUSY;
result = do_kexec_load(entry, nr_segments, ksegments, flags);
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return result;
}
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index c19c0dad1ebe..71682a33b1ba 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -45,7 +45,7 @@
#include <crypto/sha.h>
#include "kexec_internal.h"
-DEFINE_MUTEX(kexec_mutex);
+static DEFINE_MUTEX(kexec_mutex);
/* Per cpu memory for storing cpu states in case of system crash. */
note_buf_t __percpu *crash_notes;
@@ -70,6 +70,43 @@ struct resource crashk_low_res = {
.desc = IORES_DESC_CRASH_KERNEL
};
+void kexec_lock(void)
+{
+ /*
+ * LOCK kexec_mutex cmpxchg(&panic_cpu, INVALID, cpu)
+ * MB MB
+ * panic_cpu == INVALID kexec_mutex == LOCKED
+ *
+ * Ensures either we observe the cmpxchg, or crash_kernel() observes
+ * our lock acquisition.
+ */
+ mutex_lock(&kexec_mutex);
+ smp_mb();
+ atomic_cond_read_acquire(&panic_cpu, VAL == PANIC_CPU_INVALID);
+}
+
+int kexec_trylock(void) {
+ if (!mutex_trylock(&kexec_mutex)) {
+ return 0;
+ }
+ smp_mb();
+ if (atomic_read(&panic_cpu) != PANIC_CPU_INVALID) {
+ mutex_unlock(&kexec_mutex);
+ return 0;
+ }
+ return 1;
+}
+
+void kexec_unlock(void)
+{
+ mutex_unlock(&kexec_mutex);
+}
+
+int kexec_is_locked(void)
+{
+ return mutex_is_locked(&kexec_mutex);
+}
+
int kexec_should_crash(struct task_struct *p)
{
/*
@@ -943,24 +980,15 @@ int kexec_load_disabled;
*/
void __noclone __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
- * we are using after a panic on a different cpu.
- *
- * If the crash kernel was not located in a fixed area
- * of memory the xchg(&kexec_crash_image) would be
- * sufficient. But since I reuse the memory...
- */
- if (mutex_trylock(&kexec_mutex)) {
- if (kexec_crash_image) {
- struct pt_regs fixed_regs;
-
- crash_setup_regs(&fixed_regs, regs);
- crash_save_vmcoreinfo();
- machine_crash_shutdown(&fixed_regs);
- machine_kexec(kexec_crash_image);
- }
- mutex_unlock(&kexec_mutex);
+ BUG_ON(atomic_read(&panic_cpu) != raw_smp_processor_id());
+
+ if (!kexec_is_locked() && kexec_crash_image) {
+ struct pt_regs fixed_regs;
+
+ crash_setup_regs(&fixed_regs, regs);
+ crash_save_vmcoreinfo();
+ machine_crash_shutdown(&fixed_regs);
+ machine_kexec(kexec_crash_image);
}
}
STACK_FRAME_NON_STANDARD(__crash_kexec);
@@ -977,9 +1005,11 @@ void crash_kexec(struct pt_regs *regs)
this_cpu = raw_smp_processor_id();
old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, this_cpu);
if (old_cpu == PANIC_CPU_INVALID) {
- /* This is the 1st CPU which comes here, so go ahead. */
- printk_safe_flush_on_panic();
- __crash_kexec(regs);
+ if (!kexec_is_locked()) {
+ /* This is the 1st CPU which comes here, so go ahead. */
+ printk_safe_flush_on_panic();
+ __crash_kexec(regs);
+ }
/*
* Reset panic_cpu to allow another panic()/crash_kexec()
@@ -993,10 +1023,10 @@ size_t crash_get_memory_size(void)
{
size_t size = 0;
- mutex_lock(&kexec_mutex);
+ kexec_lock();
if (crashk_res.end != crashk_res.start)
size = resource_size(&crashk_res);
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return size;
}
@@ -1016,7 +1046,7 @@ int crash_shrink_memory(unsigned long new_size)
unsigned long old_size;
struct resource *ram_res;
- mutex_lock(&kexec_mutex);
+ kexec_lock();
if (kexec_crash_image) {
ret = -ENOENT;
@@ -1054,7 +1084,7 @@ int crash_shrink_memory(unsigned long new_size)
insert_resource(&iomem_resource, ram_res);
unlock:
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return ret;
}
@@ -1126,7 +1156,7 @@ int kernel_kexec(void)
{
int error = 0;
- if (!mutex_trylock(&kexec_mutex))
+ if (!kexec_trylock())
return -EBUSY;
if (!kexec_image) {
error = -EINVAL;
@@ -1203,7 +1233,7 @@ int kernel_kexec(void)
#endif
Unlock:
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return error;
}
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index ca40bef75a61..d40b0aedc187 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -362,7 +362,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd,
int, initrd_fd,
image = NULL;
- if (!mutex_trylock(&kexec_mutex))
+ if (!kexec_trylock())
return -EBUSY;
dest_image = &kexec_image;
@@ -434,7 +434,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd,
int, initrd_fd,
if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image)
arch_kexec_protect_crashkres();
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
kimage_free(image);
return ret;
}
diff --git a/kernel/kexec_internal.h b/kernel/kexec_internal.h
index 39d30ccf8d87..2c1683cb1082 100644
--- a/kernel/kexec_internal.h
+++ b/kernel/kexec_internal.h
@@ -15,7 +15,11 @@ int kimage_is_destination_range(struct kimage *image,
int machine_kexec_post_load(struct kimage *image);
-extern struct mutex kexec_mutex;
+void kexec_lock(void);
+int kexec_trylock(void);
+void kexec_unlock(void);
+int kexec_is_locked(void);
+
#ifdef CONFIG_KEXEC_FILE
#include <linux/purgatory.h>
--
2.25.1
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