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From:   Valentin Schneider <vschneid@redhat.com>
To:     kexec@lists.infradead.org, linux-rt-users@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Eric Biederman <ebiederm@xmission.com>,
        Arnd Bergmann <arnd@arndb.de>, Petr Mladek <pmladek@suse.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Juri Lelli <jlelli@redhat.com>,
        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Subject: [PATCH v4 1/2] kexec: Turn all kexec_mutex acquisitions into trylocks
Date:   Thu, 30 Jun 2022 23:32:57 +0100
Message-Id: <20220630223258.4144112-2-vschneid@redhat.com>
In-Reply-To: <20220630223258.4144112-1-vschneid@redhat.com>
References: <20220630223258.4144112-1-vschneid@redhat.com>
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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