Re: [RFC 1/5] tty/sysrq: Make sysrq handler NMI aware

[Sumit Garg]

+ Peter (author of irq_work.c)

On Thu, 13 Aug 2020 at 05:30, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
> >
> > In a future patch we will add support to the serial core to make it
> > possible to trigger a magic sysrq from an NMI context. Prepare for this
> > by marking some sysrq actions as NMI safe. Safe actions will be allowed
> > to run from NMI context whilst that cannot run from an NMI will be queued
> > as irq_work for later processing.
> >
> > A particular sysrq handler is only marked as NMI safe in case the handler
> > isn't contending for any synchronization primitives as in NMI context
> > they are expected to cause deadlocks. Note that the debug sysrq do not
> > contend for any synchronization primitives. It does call kgdb_breakpoint()
> > to provoke a trap but that trap handler should be NMI safe on
> > architectures that implement an NMI.
> >
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >  drivers/tty/sysrq.c       | 33 ++++++++++++++++++++++++++++++++-
> >  include/linux/sysrq.h     |  1 +
> >  kernel/debug/debug_core.c |  1 +
> >  3 files changed, 34 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> > index 7c95afa9..8017e33 100644
> > --- a/drivers/tty/sysrq.c
> > +++ b/drivers/tty/sysrq.c
> > @@ -50,6 +50,8 @@
> >  #include <linux/syscalls.h>
> >  #include <linux/of.h>
> >  #include <linux/rcupdate.h>
> > +#include <linux/irq_work.h>
> > +#include <linux/kfifo.h>
> >
> >  #include <asm/ptrace.h>
> >  #include <asm/irq_regs.h>
> > @@ -111,6 +113,7 @@ static const struct sysrq_key_op sysrq_loglevel_op = {
> >         .help_msg       = "loglevel(0-9)",
> >         .action_msg     = "Changing Loglevel",
> >         .enable_mask    = SYSRQ_ENABLE_LOG,
> > +       .nmi_safe       = true,
> >  };
> >
> >  #ifdef CONFIG_VT
> > @@ -157,6 +160,7 @@ static const struct sysrq_key_op sysrq_crash_op = {
> >         .help_msg       = "crash(c)",
> >         .action_msg     = "Trigger a crash",
> >         .enable_mask    = SYSRQ_ENABLE_DUMP,
> > +       .nmi_safe       = true,
> >  };
> >
> >  static void sysrq_handle_reboot(int key)
> > @@ -170,6 +174,7 @@ static const struct sysrq_key_op sysrq_reboot_op = {
> >         .help_msg       = "reboot(b)",
> >         .action_msg     = "Resetting",
> >         .enable_mask    = SYSRQ_ENABLE_BOOT,
> > +       .nmi_safe       = true,
> >  };
> >
> >  const struct sysrq_key_op *__sysrq_reboot_op = &sysrq_reboot_op;
> > @@ -217,6 +222,7 @@ static const struct sysrq_key_op sysrq_showlocks_op = {
> >         .handler        = sysrq_handle_showlocks,
> >         .help_msg       = "show-all-locks(d)",
> >         .action_msg     = "Show Locks Held",
> > +       .nmi_safe       = true,
> >  };
> >  #else
> >  #define sysrq_showlocks_op (*(const struct sysrq_key_op *)NULL)
> > @@ -289,6 +295,7 @@ static const struct sysrq_key_op sysrq_showregs_op = {
> >         .help_msg       = "show-registers(p)",
> >         .action_msg     = "Show Regs",
> >         .enable_mask    = SYSRQ_ENABLE_DUMP,
> > +       .nmi_safe       = true,
> >  };
> >
> >  static void sysrq_handle_showstate(int key)
> > @@ -326,6 +333,7 @@ static const struct sysrq_key_op sysrq_ftrace_dump_op = {
> >         .help_msg       = "dump-ftrace-buffer(z)",
> >         .action_msg     = "Dump ftrace buffer",
> >         .enable_mask    = SYSRQ_ENABLE_DUMP,
> > +       .nmi_safe       = true,
> >  };
> >  #else
> >  #define sysrq_ftrace_dump_op (*(const struct sysrq_key_op *)NULL)
> > @@ -538,6 +546,23 @@ static void __sysrq_put_key_op(int key, const struct sysrq_key_op *op_p)
> >                  sysrq_key_table[i] = op_p;
> >  }
> >
> > +#define SYSRQ_NMI_FIFO_SIZE    64
> > +static DEFINE_KFIFO(sysrq_nmi_fifo, int, SYSRQ_NMI_FIFO_SIZE);
>
> A 64-entry FIFO seems excessive. Quite honestly even a FIFO seems a
> bit excessive and it feels like if two sysrqs were received in super
> quick succession that it would be OK to just process the first one.  I
> guess if it simplifies the processing to have a FIFO then it shouldn't
> hurt, but no need for 64 entries.
>

Okay, would a 2-entry FIFO work here? As here we need a FIFO to pass
on the key parameter.

>
> > +static void sysrq_do_nmi_work(struct irq_work *work)
> > +{
> > +       const struct sysrq_key_op *op_p;
> > +       int key;
> > +
> > +       while (kfifo_out(&sysrq_nmi_fifo, &key, 1)) {
> > +               op_p = __sysrq_get_key_op(key);
> > +               if (op_p)
> > +                       op_p->handler(key);
> > +       }
>
> Do you need to manage "suppress_printk" in this function?  Do you need
> to call rcu_sysrq_start() and rcu_read_lock()?

Ah I missed those. Will add them here instead.

>
> If so, how do you prevent racing between the mucking we're doing with
> these things and the mucking that the NMI does with them?

IIUC, here you meant to highlight the race while scheduled sysrq is
executing in IRQ context and we receive a new sysrq in NMI context,
correct? If yes, this seems to be a trickier situation. I think the
appropriate way to handle it would be to deny any further sysrq
handling until the prior sysrq handling is complete, your views?

>
>
> > +}
> > +
> > +static DEFINE_IRQ_WORK(sysrq_nmi_work, sysrq_do_nmi_work);
> > +
> >  void __handle_sysrq(int key, bool check_mask)
> >  {
> >         const struct sysrq_key_op *op_p;
> > @@ -568,7 +593,13 @@ void __handle_sysrq(int key, bool check_mask)
> >                 if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
> >                         pr_info("%s\n", op_p->action_msg);
> >                         console_loglevel = orig_log_level;
> > -                       op_p->handler(key);
> > +
> > +                       if (in_nmi() && !op_p->nmi_safe) {
> > +                               kfifo_in(&sysrq_nmi_fifo, &key, 1);
>
> Rather than kfifo_in() and kfifo_out(), I think you can use
> kfifo_put() and kfifo_get().  As I understand it those just get/put
> one element which is what you want.

Okay, will use kfifo_put() and kfifo_get() here instead.

>
>
> > +                               irq_work_queue(&sysrq_nmi_work);
>
> Wishful thinking, but (as far as I can tell) irq_work_queue() only
> queues work on the CPU running the NMI.  I don't have lots of NMI
> experience, but any chance there is a variant that will queue work on
> any CPU?  Then sysrq handlers that aren't NMI aware will be more
> likely to work.
>

Unfortunately, queuing work on other CPUs isn't safe in NMI context,
see this warning [1]. The comment mentions:

/* Arch remote IPI send/receive backend aren't NMI safe */

Peter,

Can you throw some light here as to why it isn't considered NMI-safe
to send remote IPI in NMI context? Is it an arch specific limitation?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/irq_work.c#n103

-Sumit

>
>
>
> > +                       } else {
> > +                               op_p->handler(key);
> > +                       }
> >                 } else {
> >                         pr_info("This sysrq operation is disabled.\n");
> >                         console_loglevel = orig_log_level;
> > diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
> > index 3a582ec..630b5b9 100644
> > --- a/include/linux/sysrq.h
> > +++ b/include/linux/sysrq.h
> > @@ -34,6 +34,7 @@ struct sysrq_key_op {
> >         const char * const help_msg;
> >         const char * const action_msg;
> >         const int enable_mask;
> > +       const bool nmi_safe;
> >  };
> >
> >  #ifdef CONFIG_MAGIC_SYSRQ
> > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> > index 9e59347..2b51173 100644
> > --- a/kernel/debug/debug_core.c
> > +++ b/kernel/debug/debug_core.c
> > @@ -943,6 +943,7 @@ static const struct sysrq_key_op sysrq_dbg_op = {
> >         .handler        = sysrq_handle_dbg,
> >         .help_msg       = "debug(g)",
> >         .action_msg     = "DEBUG",
> > +       .nmi_safe       = true,
> >  };
> >  #endif
> >
> > --
> > 2.7.4
> >