[V2 PATCH 2/3] kexec: Fix race between panic() and crash_kexec() called directly

[Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>]

Currently, panic() and crash_kexec() can be called at the same time.
For example (x86 case):

CPU 0:
  oops_end()
    crash_kexec()
      mutex_trylock() // acquired
        nmi_shootdown_cpus() // stop other cpus

CPU 1:
  panic()
    crash_kexec()
      mutex_trylock() // failed to acquire
    smp_send_stop() // stop other cpus
    infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump
fails.

In another case:

CPU 0:
  oops_end()
    crash_kexec()
      mutex_trylock() // acquired
        <NMI>
        io_check_error()
          panic()
            crash_kexec()
              mutex_trylock() // failed to acquire
            infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude
others by using atomic_t panicking_cpu.

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock
  to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/kexec.c |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/kernel/kexec.c b/kernel/kexec.c
index a785c10..ca40a19 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void)
 
 void crash_kexec(struct pt_regs *regs)
 {
+	int old_cpu, this_cpu;
+
+	/*
+	 * `old_cpu == -1' means we are the first comer and crash_kexec()
+	 * was called without entering panic().
+	 * `old_cpu == this_cpu' means crash_kexec() was called from panic().
+	 */
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panicking_cpu, -1, this_cpu);
+	if (old_cpu != -1 && old_cpu != this_cpu)
+		return;
+
 	/* 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.
@@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs)
 		}
 		mutex_unlock(&kexec_mutex);
 	}
+
+	/*
+	 * If we came here from panic(), we have to keep panicking_cpu
+	 * to prevent other cpus from entering panic().  Otherwise,
+	 * resetting it so that other cpus can enter panic()/crash_kexec().
+	 */
+	if (old_cpu == this_cpu)
+		atomic_set(&panicking_cpu, -1);
 }
 
 size_t crash_get_memory_size(void)