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


> +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()?

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?


> +}
> +
> +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.


> + 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.




> + } 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
>

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