On 09/25/23 at 09:59am, Eric DeVolder wrote:
>
>
> On 9/24/23 22:07, Baoquan He wrote:
> > Eric reported that handling corresponding crash hotplug event can be
> > failed easily when many memory hotplug event are notified in a short
> > period. They failed because failing to take __kexec_lock.
> >
> > =======
> > [ 78.714569] Fallback order for Node 0: 0
> > [ 78.714575] Built 1 zonelists, mobility grouping on. Total pages: 1817886
> > [ 78.717133] Policy zone: Normal
> > [ 78.724423] crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate
> > [ 78.727207] crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate
> > [ 80.056643] PEFILE: Unsigned PE binary
> > =======
> >
> > The memory hotplug events are notified very quickly and very many,
> > while the handling of crash hotplug is much slower relatively. So the
> > atomic variable __kexec_lock and kexec_trylock() can't guarantee the
> > serialization of crash hotplug handling.
> >
> > Here, add a new mutex lock __crash_hotplug_lock to serialize crash
> > hotplug handling specifically. This doesn't impact the usage of
> > __kexec_lock.
> >
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> > v1->v2:
> > - Move mutex lock definition into CONFIG_CRASH_HOTPLUG ifdeffery
> > scope in kernel/crash_core.c because the lock is only needed and
> > used in that scope. Suggested by Eric.
> >
> > kernel/crash_core.c | 14 ++++++++++++++
> > 1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index 03a7932cde0a..5951d6366b72 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -739,6 +739,17 @@ subsys_initcall(crash_notes_memory_init);
> > #undef pr_fmt
> > #define pr_fmt(fmt) "crash hp: " fmt
> > +/*
> > + * Different than kexec/kdump loading/unloading/jumping/shrinking which
> > + * usually rarely happen, there will be many crash hotplug events notified
> > + * during one short period, e.g one memory board is hot added and memory
> > + * regions are online. So mutex lock __crash_hotplug_lock is used to
> > + * serialize the crash hotplug handling specifically.
> > + */
> > +DEFINE_MUTEX(__crash_hotplug_lock);
> > +#define crash_hotplug_lock() mutex_lock(&__crash_hotplug_lock)
> > +#define crash_hotplug_unlock() mutex_unlock(&__crash_hotplug_lock)
> > +
> > /*
> > * This routine utilized when the crash_hotplug sysfs node is read.
> > * It reflects the kernel's ability/permission to update the crash
> > @@ -783,9 +794,11 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
> > {
> > struct kimage *image;
> > + crash_hotplug_lock();
> > /* Obtain lock while changing crash information */
> > if (!kexec_trylock()) {
> > pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> > + crash_hotplug_unlock();
> > return;
> > }
> > @@ -852,6 +865,7 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
> > out:
> > /* Release lock now that update complete */
> > kexec_unlock();
> > + crash_hotplug_unlock();
> > }
> > static int crash_memhp_notifier(struct notifier_block *nb, unsigned long val, void *v)
>
> The crash_check_update_elfcorehdr() also has kexec_trylock() and needs similar treatment.
> Userspace (ie udev rule processing) and kernel (crash hotplug infrastrucutre) need to be
> protected/serialized from one another.
You are right. I didn't consider the kexec_load interface. There's a
tiny racing window which we still need to avoid. V3 will be posted.
Thanks.
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