Hi,

On Mon, Aug 17, 2020 at 7:08 AM Sumit Garg [off-list ref] wrote:

On Fri, 14 Aug 2020 at 20:27, Doug Anderson [off-list ref] wrote:

quoted

Hi,

On Fri, Aug 14, 2020 at 12:24 AM Sumit Garg [off-list ref] wrote:

quoted

+ Peter (author of irq_work.c)

On Thu, 13 Aug 2020 at 05:30, Doug Anderson [off-list ref] wrote:

quoted

Hi,

On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg [off-list ref] wrote:

quoted

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 <redacted>
---
 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).

Typically 1 is considered to be a power of 2 since 2^0 = 1.

...ah, but it appears that size < 2 is not allowed.  Oh well.

I think you can use kfifo_reset_out().

This _seems_ OK to me since I'd imagine kfifo_is_empty() is as safe
for the writer to do as kfifo_is_full() is and kfifo_is_full() is part
of kfifo_put().

I guess there's no better synchronism mechanism that we can use?

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