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[209.85.217.47]) by smtp.gmail.com with ESMTPSA id j195sm3582369vke.18.2020.08.17.10.19.21 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 Aug 2020 10:19:21 -0700 (PDT) Received: by mail-vs1-f47.google.com with SMTP id a1so8660279vsp.4 for ; Mon, 17 Aug 2020 10:19:21 -0700 (PDT) X-Received: by 2002:a67:f44f:: with SMTP id r15mr8546380vsn.42.1597684760494; Mon, 17 Aug 2020 10:19:20 -0700 (PDT) MIME-Version: 1.0 References: <1595333413-30052-1-git-send-email-sumit.garg@linaro.org> <1595333413-30052-2-git-send-email-sumit.garg@linaro.org> In-Reply-To: From: Doug Anderson Date: Mon, 17 Aug 2020 10:19:08 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [RFC 1/5] tty/sysrq: Make sysrq handler NMI aware To: Sumit Garg Cc: Peter Zijlstra , Greg Kroah-Hartman , Daniel Thompson , linux-serial@vger.kernel.org, kgdb-bugreport@lists.sourceforge.net, Jiri Slaby , Russell King - ARM Linux , Jason Wessel , LKML , Linux ARM Content-Type: text/plain; charset="UTF-8" Sender: linux-serial-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org Hi, On Mon, Aug 17, 2020 at 7:08 AM Sumit Garg wrote: > > On Fri, 14 Aug 2020 at 20:27, Doug Anderson wrote: > > > > Hi, > > > > On Fri, Aug 14, 2020 at 12:24 AM Sumit Garg wrote: > > > > > > + Peter (author of irq_work.c) > > > > > > On Thu, 13 Aug 2020 at 05:30, Doug Anderson wrote: > > > > > > > > Hi, > > > > > > > > On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg 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 > > > > > --- > > > > > 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 > > > > > #include > > > > > #include > > > > > +#include > > > > > +#include > > > > > > > > > > #include > > > > > #include > > > > > @@ -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(). > +} > + > +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? > + > 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