Re: [PATCH] rxrpc: struct mutex cannot be used for rxrpc_call::user_mutex

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 18, 2019 at 12:28:01PM -0800, Davidlohr Bueso wrote:
> Hmm so fyi __crash_kexec() is another one, but can be called in hard-irq, and
> it's extremely obvious that the trylock+unlock occurs in the same context.

Hurmph, that unlock 'never' happens if I read it right :-) Still,
something like the below ought to cure it I suppose.

> It would be nice to automate this...

Automate what exactly? We'll stick your WARN back in on the next round.


---
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 15d70a90b50d..2faf2ec33032 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -47,6 +47,26 @@
 
 DEFINE_MUTEX(kexec_mutex);
 
+static 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_load_acquire(&panic_cpu, VAL == PANIC_CPU_INVALID);
+}
+
+static void kexec_unlock(void)
+{
+	mutex_unlock(&kexec_mutex);
+}
+
 /* Per cpu memory for storing cpu states in case of system crash. */
 note_buf_t __percpu *crash_notes;
 
@@ -937,24 +957,13 @@ 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);
+	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);
 	}
 }
 STACK_FRAME_NON_STANDARD(__crash_kexec);
@@ -973,7 +982,11 @@ void crash_kexec(struct pt_regs *regs)
 	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);
+		/*
+		 * Orders against kexec_lock(), see the comment there.
+		 */
+		if (!mutex_is_locked(&kexec_mutex))
+			__crash_kexec(regs);
 
 		/*
 		 * Reset panic_cpu to allow another panic()/crash_kexec()
@@ -987,10 +1000,10 @@ 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;
 }
 
@@ -1010,7 +1023,7 @@ 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;
@@ -1048,7 +1061,7 @@ int crash_shrink_memory(unsigned long new_size)
 	insert_resource(&iomem_resource, ram_res);
 
 unlock:
-	mutex_unlock(&kexec_mutex);
+	kexec_unlock();
 	return ret;
 }