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

Steven Rostedt
2015-11-24T20:36:12Z

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@hitachi.com>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> ---
> 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);
>
>