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

河合英宏 / KAWAI，HIDEHIRO
2015-11-25T06:28:20Z

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> 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?

Yes, but the important thing is that CPU 1 stops CPU 0 which is
only CPU processing crash_ kexec routines.

> >
> > 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.

In this patch, an exclusive execution control with panic_cpu
is added to crash_kexec(). When crash_kexec() is called from
panic(), we don't need to check panic_cpu because we have already
held the exclusive control. So, __crash_kexec() is used here
to bypass it.

Of course, we can call crash_kexec() here, and crash_kexec()
checks if panic_cpu is equal to the current CPU number, and
if so, continues to process crash_kexec() routines.
This was done in older version of this patch series, but
Peter received a wrong impression about checking if panic_cpu
is equal to the current CPU number; it implies that it permits
recursive call of crash_kexec() (actually recursive call of
crash_kexec() can't happen).

Anyway, I'll add some comments.

Regards,

--
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group

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