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    Subject[V6 PATCH 3/6] kexec: Fix race between panic() and crash_kexec() called directly
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    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 panic_cpu.

    V6:
    - Use PANIC_CPU_INVALID instead of hard-coded -1
    - Add comments about __crash_kexec()

    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 | 30 +++++++++++++++++++++++++++++-
    kernel/panic.c | 4 ++--
    3 files changed, 33 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..c823f30 100644
    --- a/kernel/kexec_core.c
    +++ b/kernel/kexec_core.c
    @@ -853,7 +853,12 @@ 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(). This function is called
    + * only when panic_cpu holds the current CPU number; this is the only CPU
    + * which processes crash_kexec routines.
    + */
    +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 +881,29 @@ 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, PANIC_CPU_INVALID, this_cpu);
    + if (old_cpu == PANIC_CPU_INVALID) {
    + /* 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, PANIC_CPU_INVALID);
    + }
    +}
    +
    size_t crash_get_memory_size(void)
    {
    size_t size = 0;
    diff --git a/kernel/panic.c b/kernel/panic.c
    index 3d6c3f1..81a0a3d 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); /* bypass panic_cpu check */

    /*
    * 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); /* bypass panic_cpu check */

    bust_spinlocks(0);




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