by Sumit Garg

[permalink] [raw]

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

On Mon, 17 Aug 2020 at 22:49, Doug Anderson <[email protected]> wrote:
>
> Hi,
>
> On Mon, Aug 17, 2020 at 7:08 AM Sumit Garg <[email protected]> wrote:
> >
> > On Fri, 14 Aug 2020 at 20:27, Doug Anderson <[email protected]> wrote:
> > >
> > > Hi,
> > >
> > > On Fri, Aug 14, 2020 at 12:24 AM Sumit Garg <[email protected]> wrote:
> > > >
> > > > + Peter (author of irq_work.c)
> > > >
> > > > On Thu, 13 Aug 2020 at 05:30, Doug Anderson <[email protected]> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg <[email protected]> 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 <[email protected]>
> > > > > > ---
> > > > > > 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.
>
>
> > > > > > +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))
> > + ;
>
> I think you can use kfifo_reset_out().
>

Okay, it sounds safe as well when used concurrently with
kfifo_is_empty(). Will use it instead.

>
> > +}
> > +
> > +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;
>
> 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?
>

Yeah, unless someone else has a better idea.

-Sumit

>
> > +
> > 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