[PATCH v4 1/2] kexec: Turn all kexec_mutex acquisitions into trylocks

Valentin Schneider
2022-06-30T22:33:18Z

Most acquistions of kexec_mutex are done via mutex_trylock() - those were a
direct "translation" from:

8c5a1cf0ad3a ("kexec: use a mutex for locking rather than xchg()")

there has however been two additions since then that use mutex_lock():
crash_get_memory_size() and crash_shrink_memory().

A later commit will replace said mutex with an atomic variable, and locking
operations will become atomic_cmpxchg(). Rather than having those
mutex_lock() become while (atomic_cmpxchg(&lock, 0, 1)), turn them into
trylocks that can return -EBUSY on acquisition failure.

This does halve the printable size of the crash kernel, but that's still
neighbouring 2G for 32bit kernels which should be ample enough.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
include/linux/kexec.h | 2 +-
kernel/kexec_core.c | 12 ++++++++----
kernel/ksysfs.c | 7 ++++++-
3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index ce6536f1d269..54d7030d3c41 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -390,7 +390,7 @@ extern int kexec_load_disabled;
extern bool kexec_in_progress;

int crash_shrink_memory(unsigned long new_size);
-size_t crash_get_memory_size(void);
+ssize_t crash_get_memory_size(void);
void crash_free_reserved_phys_range(unsigned long begin, unsigned long end);

void arch_kexec_protect_crashkres(void);
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 4d34c78334ce..16370926b21a 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1009,13 +1009,16 @@ void crash_kexec(struct pt_regs *regs)
}
}

-size_t crash_get_memory_size(void)
+ssize_t crash_get_memory_size(void)
{
- size_t size = 0;
+ ssize_t size = 0;
+
+ if (!mutex_trylock(&kexec_mutex))
+ return -EBUSY;

- mutex_lock(&kexec_mutex);
if (crashk_res.end != crashk_res.start)
size = resource_size(&crashk_res);
+
mutex_unlock(&kexec_mutex);
return size;
}
@@ -1036,7 +1039,8 @@ int crash_shrink_memory(unsigned long new_size)
unsigned long old_size;
struct resource *ram_res;

- mutex_lock(&kexec_mutex);
+ if (!mutex_trylock(&kexec_mutex))
+ return -EBUSY;

if (kexec_crash_image) {
ret = -ENOENT;
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index b1292a57c2a5..65dba9076f31 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -105,7 +105,12 @@ KERNEL_ATTR_RO(kexec_crash_loaded);
static ssize_t kexec_crash_size_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
{
- return sprintf(buf, "%zu\n", crash_get_memory_size());
+ ssize_t size = crash_get_memory_size();
+
+ if (size < 0)
+ return size;
+
+ return sprintf(buf, "%zd\n", size);
}
static ssize_t kexec_crash_size_store(struct kobject *kobj,
struct kobj_attribute *attr,
--
2.31.1