Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

[Steven Rostedt]

On Fri, Nov 20, 2015 at 06:36:48PM +0900, Hidehiro Kawai wrote:
> 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.

So the smp_send_stop() stops CPU 0 from calling nmi_shootdown_cpus(), right?

> 
> 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.

I'm trying to see how this patch fixes this case.

> 
> To fix this problem, this patch changes crash_kexec() to exclude
> others by using atomic_t panic_cpu.
> 
> V5:
> - Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
> - Replace atomic_xchg() with atomic_set() in crash_kexec() because
>   it is used as a release operation and there is no need of memory
>   barrier effect.  This change also removes an unused value warning
> 
> V4:
> - Use new __crash_kexec(), no exclusion check version of crash_kexec(),
>   instead of checking if panic_cpu is the current cpu or not
> 
> 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@xxxxxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> ---
>  include/linux/kexec.h |    2 ++
>  kernel/kexec_core.c   |   26 +++++++++++++++++++++++++-
>  kernel/panic.c        |    4 ++--
>  3 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index d140b1e..7b68d27 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -237,6 +237,7 @@ extern int kexec_purgatory_get_set_symbol(struct kimage *image,
>  					  unsigned int size, bool get_value);
>  extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
>  					     const char *name);
> +extern void __crash_kexec(struct pt_regs *);
>  extern void crash_kexec(struct pt_regs *);
>  int kexec_should_crash(struct task_struct *);
>  void crash_save_cpu(struct pt_regs *regs, int cpu);
> @@ -332,6 +333,7 @@ int __weak arch_kexec_apply_relocations(const Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>  #else /* !CONFIG_KEXEC_CORE */
>  struct pt_regs;
>  struct task_struct;
> +static inline void __crash_kexec(struct pt_regs *regs) { }
>  static inline void crash_kexec(struct pt_regs *regs) { }
>  static inline int kexec_should_crash(struct task_struct *p) { return 0; }
>  #define kexec_in_progress false
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 11b64a6..9d097f5 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -853,7 +853,8 @@ struct kimage *kexec_image;
>  struct kimage *kexec_crash_image;
>  int kexec_load_disabled;
>  
> -void crash_kexec(struct pt_regs *regs)
> +/* No panic_cpu check version of crash_kexec */
> +void __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
> @@ -876,6 +877,29 @@ void crash_kexec(struct pt_regs *regs)
>  	}
>  }
>  
> +void crash_kexec(struct pt_regs *regs)
> +{
> +	int old_cpu, this_cpu;
> +
> +	/*
> +	 * Only one CPU is allowed to execute the crash_kexec() code as with
> +	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
> +	 * may stop each other.  To exclude them, we use panic_cpu here too.
> +	 */
> +	this_cpu = raw_smp_processor_id();
> +	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> +	if (old_cpu == -1) {
> +		/* This is the 1st CPU which comes here, so go ahead. */
> +		__crash_kexec(regs);
> +
> +		/*
> +		 * Reset panic_cpu to allow another panic()/crash_kexec()
> +		 * call.
> +		 */
> +		atomic_set(&panic_cpu, -1);
> +	}
> +}
> +
>  size_t crash_get_memory_size(void)
>  {
>  	size_t size = 0;
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 4fce2be..5d0b807 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -138,7 +138,7 @@ void panic(const char *fmt, ...)
>  	 * the "crash_kexec_post_notifiers" option to the kernel.
>  	 */
>  	if (!crash_kexec_post_notifiers)
> -		crash_kexec(NULL);
> +		__crash_kexec(NULL);

Why call the __crash_kexec() version and not just crash_kexec() here.
This needs to be documented.

>  
>  	/*
>  	 * Note smp_send_stop is the usual smp shutdown function, which
> @@ -163,7 +163,7 @@ void panic(const char *fmt, ...)
>  	 * more unstable, it can increase risks of the kdump failure too.
>  	 */
>  	if (crash_kexec_post_notifiers)
> -		crash_kexec(NULL);
> +		__crash_kexec(NULL);

ditto.

-- Steve

>  
>  	bust_spinlocks(0);
>  
> 
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