kexec: turn all kexec_mutex acquisitions into trylocks

Patch series "kexec, panic: Making crash_kexec() NMI safe", v4.


This patch (of 2):

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

  8c5a1cf0ad ("kexec: use a mutex for locking rather than xchg()")

there have 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.

Link: https://lkml.kernel.org/r/20220630223258.4144112-1-vschneid@redhat.com
Link: https://lkml.kernel.org/r/20220630223258.4144112-2-vschneid@redhat.com
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Juri Lelli <jlelli@redhat.com>
Cc: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/kexec.h

@@ -427,7 +427,7 @@ 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);
 
 #ifndef arch_kexec_protect_crashkres
 /*

kernel/kexec_core.c

@@ -1004,13 +1004,16 @@ 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;
 }
@@ -1022,7 +1025,8 @@ 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;

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,