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

[Sumit Garg <sumit.garg@linaro.org>]

On Fri, 14 Aug 2020 at 20:27, Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Fri, Aug 14, 2020 at 12:24 AM Sumit Garg <sumit.garg@linaro.org> wrote:
> >
> > + Peter (author of irq_work.c)
> >
> > On Thu, 13 Aug 2020 at 05:30, Doug Anderson <dianders@chromium.org> wrote:
> > >
> > > Hi,
> > >
> > > On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg <sumit.garg@linaro.org> 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@linaro.org>
> > > > ---
> > > >  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.
>
> ...or even a 1-entry FIFO if that makes sense?
>

Yes it would make sense but unfortunately not supported by kfifo
(size: power of 2).

>
> > > > +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?
>
> The problem is that in some cases you're running NMIs directly at FIQ
> time and other cases you're running them at IRQ time.  So you
> definitely can't just move it to NMI.
>
> Skipping looking for other SYSRQs until the old one is complete sounds
> good to me.  Again my ignorance will make me sound like a fool,
> probably, but can you use the kfifo as a form of mutual exclusion?  If
> you have a 1-entry kfifo, maybe:
>
> 1. First try to add to the "FIFO".  If it fails (out of space) then a
> sysrq is in progress.  Ignore this one.
> 2. Decide if you're NMI-safe or not.
> 3. If NMI safe, modify "suppress_printk", call rcu functions, then
> call the handler.  Restore suppress_printk and then dequeue from FIFO.
> 4. If not-NMI safe, the irq worker would "peek" into the FIFO, do its
> work (wrapped with "suppress_printk" and the like), and not dequeue
> until it's done.
>
> In the above you'd use the FIFO as a locking mechanism.  I don't know
> if that's a valid use of it or if there is a better NMI-safe mechanism
> for this.  I think the kfifo docs talk about only one reader and one
> writer and here we have two readers, so maybe it's illegal.  It also
> seems weird to have a 1-entry "FIFO" and feels like there's probably a
> better data structure for this.

Thanks for your suggestions. Have a look at below implementation, I
have used 2-entry fifo but only single entry used for locking
mechanism:

@@ -538,6 +546,39 @@ 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    2
+static DEFINE_KFIFO(sysrq_nmi_fifo, int, SYSRQ_NMI_FIFO_SIZE);
+
+static void sysrq_do_nmi_work(struct irq_work *work)
+{
+       const struct sysrq_key_op *op_p;
+       int orig_suppress_printk;
+       int key;
+
+       orig_suppress_printk = suppress_printk;
+       suppress_printk = 0;
+
+       rcu_sysrq_start();
+       rcu_read_lock();
+
+       if (kfifo_peek(&sysrq_nmi_fifo, &key)) {
+               op_p = __sysrq_get_key_op(key);
+               if (op_p)
+                       op_p->handler(key);
+       }
+
+       rcu_read_unlock();
+       rcu_sysrq_end();
+
+       suppress_printk = orig_suppress_printk;
+
+       /* Pop contents from fifo if any */
+       while (kfifo_get(&sysrq_nmi_fifo, &key))
+               ;
+}
+
+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;
+}
+
+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;
@@ -545,6 +586,10 @@ void __handle_sysrq(int key, bool check_mask)
        int orig_suppress_printk;
        int i;

+       /* Skip sysrq handling if one already in progress */
+       if (!kfifo_is_empty(&sysrq_nmi_fifo))
+               return;
+
        orig_suppress_printk = suppress_printk;
        suppress_printk = 0;

@@ -568,7 +613,13 @@ 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_put(&sysrq_nmi_fifo, key);
+                               irq_work_queue(&sysrq_nmi_work);
+                       } else {
+                               op_p->handler(key);
+                       }
                } else {
                        pr_info("This sysrq operation is disabled.\n");
                        console_loglevel = orig_log_level;

-Sumit