| Message ID | 1425463974-23568-5-git-send-email-daniel.thompson@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, 2015-03-04 at 10:12 +0000, Daniel Thompson wrote: > Currently there is a quite a pile of code sitting in > arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. > The code is inaccessible to backtrace implementations for other > architectures, which is a shame because they would probably like to be > safe too. > > Copy this code into printk. We'll port the x86 NMI backtrace to it in a > later patch. I think this would be better as a separate file rather than increasing the bulk of printk.c
On Wed, 04 Mar 2015 08:13:21 -0800 Joe Perches <joe@perches.com> wrote: > On Wed, 2015-03-04 at 10:12 +0000, Daniel Thompson wrote: > > Currently there is a quite a pile of code sitting in > > arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. > > The code is inaccessible to backtrace implementations for other > > architectures, which is a shame because they would probably like to be > > safe too. > > > > Copy this code into printk. We'll port the x86 NMI backtrace to it in a > > later patch. > > I think this would be better as a separate file > rather than increasing the bulk of printk.c > I agree, as printk already has its own directory. Perhaps a "nmi_backtrace.c"? -- Steve
On Wed, 2015-03-04 at 11:20 -0500, Steven Rostedt wrote: > On Wed, 04 Mar 2015 08:13:21 -0800 > Joe Perches <joe@perches.com> wrote: > > > On Wed, 2015-03-04 at 10:12 +0000, Daniel Thompson wrote: > > > Currently there is a quite a pile of code sitting in > > > arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. > > > The code is inaccessible to backtrace implementations for other > > > architectures, which is a shame because they would probably like to be > > > safe too. > > > > > > Copy this code into printk. We'll port the x86 NMI backtrace to it in a > > > later patch. > > > > I think this would be better as a separate file > > rather than increasing the bulk of printk.c > > > > I agree, as printk already has its own directory. Perhaps a > "nmi_backtrace.c"? I agree on moving the code. However, after Thomas' review I made sure all the external symbols were prefixed printk_nmi and, as a result of the same review I started using CONFIG_PRINTK_NMI to enable/disable the feature). For that reason I'm much more inclined to name it "printk_nmi.c". Any objections? I know it is a somewhat generic name but I'll move the comment text that commences "This is not a generic printk() implementation and must be used with great care. In particular..." to the top of the file to make clear the limitations of this code. Daniel.
On Wed, 2015-03-04 at 16:33 +0000, Daniel Thompson wrote: > On Wed, 2015-03-04 at 11:20 -0500, Steven Rostedt wrote: > > On Wed, 04 Mar 2015 08:13:21 -0800 > > Joe Perches <joe@perches.com> wrote: > > > > > On Wed, 2015-03-04 at 10:12 +0000, Daniel Thompson wrote: > > > > Currently there is a quite a pile of code sitting in > > > > arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. > > > > The code is inaccessible to backtrace implementations for other > > > > architectures, which is a shame because they would probably like to be > > > > safe too. > > > > > > > > Copy this code into printk. We'll port the x86 NMI backtrace to it in a > > > > later patch. > > > > > > I think this would be better as a separate file > > > rather than increasing the bulk of printk.c > > > > > I agree, as printk already has its own directory. Perhaps a > > "nmi_backtrace.c"? > > I agree on moving the code. However, after Thomas' review I made sure > all the external symbols were prefixed printk_nmi and, as a result of > the same review I started using CONFIG_PRINTK_NMI to enable/disable the > feature). For that reason I'm much more inclined to name it > "printk_nmi.c". Any objections? Steven's suggestion seems more sensible. sed 's/CONFIG_PRINTK_NMI/CONFIG_PRINTK_NMI_BACKTRACE/g' sed 's/printk_nmi/printk_nmib/g' or sed 's/printk_nmi/nmi_backtrace/g' might work well.
On Wed, 2015-03-04 at 09:21 -0800, Joe Perches wrote: > On Wed, 2015-03-04 at 16:33 +0000, Daniel Thompson wrote: > > On Wed, 2015-03-04 at 11:20 -0500, Steven Rostedt wrote: > > > On Wed, 04 Mar 2015 08:13:21 -0800 > > > Joe Perches <joe@perches.com> wrote: > > > > > > > On Wed, 2015-03-04 at 10:12 +0000, Daniel Thompson wrote: > > > > > Currently there is a quite a pile of code sitting in > > > > > arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. > > > > > The code is inaccessible to backtrace implementations for other > > > > > architectures, which is a shame because they would probably like to be > > > > > safe too. > > > > > > > > > > Copy this code into printk. We'll port the x86 NMI backtrace to it in a > > > > > later patch. > > > > > > > > I think this would be better as a separate file > > > > rather than increasing the bulk of printk.c > > > > > > > I agree, as printk already has its own directory. Perhaps a > > > "nmi_backtrace.c"? > > > > I agree on moving the code. However, after Thomas' review I made sure > > all the external symbols were prefixed printk_nmi and, as a result of > > the same review I started using CONFIG_PRINTK_NMI to enable/disable the > > feature). For that reason I'm much more inclined to name it > > "printk_nmi.c". Any objections? > > Steven's suggestion seems more sensible. > > sed 's/CONFIG_PRINTK_NMI/CONFIG_PRINTK_NMI_BACKTRACE/g' > sed 's/printk_nmi/printk_nmib/g' > or > sed 's/printk_nmi/nmi_backtrace/g' > > might work well. Ok. The later rename is consistent and makes is much less likely the facility will be (accidentally) misused in future. I'll change like this.
diff --git a/include/linux/printk.h b/include/linux/printk.h index baa3f97d8ce8..7fd94e644976 100644 --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -228,6 +228,24 @@ static inline void show_regs_print_info(const char *log_lvl) } #endif +#ifdef CONFIG_PRINTK_NMI +/* + * printk_nmi_prepare/complete are called to prepare the system for + * some or all cores to issue trace from NMI. printk_nmi_complete will + * print buffered output and cannot (safely) be called from NMI. + */ +extern int printk_nmi_prepare(void); +extern void printk_nmi_complete(void); + +/* + * printk_nmi_this_cpu_begin/end are used divert/restore printk on this + * cpu. The result is the output of printk() (by this CPU) will be + * stored in temporary buffers for later printing by printk_nmi_complete. + */ +extern void printk_nmi_this_cpu_begin(void); +extern void printk_nmi_this_cpu_end(void); +#endif + extern asmlinkage void dump_stack(void) __cold; #ifndef pr_fmt diff --git a/init/Kconfig b/init/Kconfig index f5dbc6d4261b..4f00d11ef0a4 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1421,6 +1421,9 @@ config PRINTK very difficult to diagnose system problems, saying N here is strongly discouraged. +config PRINTK_NMI + bool + config BUG bool "BUG() support" if EXPERT default y diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 01cfd69c54c6..291271300cd5 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1829,6 +1829,155 @@ EXPORT_SYMBOL_GPL(vprintk_default); */ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default; +#ifdef CONFIG_PRINTK_NMI + +#define NMI_BUF_SIZE 4096 + +struct nmi_seq_buf { + unsigned char buffer[NMI_BUF_SIZE]; + struct seq_buf seq; +}; + +/* Safe printing in NMI context */ +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); + +static DEFINE_PER_CPU(printk_func_t, nmi_print_saved_print_func); + +/* "in progress" flag of NMI printing */ +static unsigned long nmi_print_flag; + +static int __init printk_nmi_init(void) +{ + struct nmi_seq_buf *s; + int cpu; + + for_each_possible_cpu(cpu) { + s = &per_cpu(nmi_print_seq, cpu); + seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE); + } + + return 0; +} +pure_initcall(printk_nmi_init); + +/* + * It is not safe to call printk() directly from NMI handlers. + * It may be fine if the NMI detected a lock up and we have no choice + * but to do so, but doing a NMI on all other CPUs to get a back trace + * can be done with a sysrq-l. We don't want that to lock up, which + * can happen if the NMI interrupts a printk in progress. + * + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes + * the content into a per cpu seq_buf buffer. Then when the NMIs are + * all done, we can safely dump the contents of the seq_buf to a printk() + * from a non NMI context. + * + * This is not a generic printk() implementation and must be used with + * great care. In particular there is a static limit on the quantity of + * data that may be emitted during NMI, only one client can be active at + * one time (arbitrated by the return value of printk_nmi_begin() and + * it is required that something at task or interrupt context be scheduled + * to issue the output. + */ +static int nmi_vprintk(const char *fmt, va_list args) +{ + struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq); + unsigned int len = seq_buf_used(&s->seq); + + seq_buf_vprintf(&s->seq, fmt, args); + return seq_buf_used(&s->seq) - len; +} + +/* + * Reserve the NMI printk mechanism. Return an error if some other component + * is already using it. + */ +int printk_nmi_prepare(void) +{ + if (test_and_set_bit(0, &nmi_print_flag)) { + /* + * If something is already using the NMI print facility we + * can't allow a second one... + */ + return -EBUSY; + } + + return 0; +} + +static void print_seq_line(struct nmi_seq_buf *s, int start, int end) +{ + const char *buf = s->buffer + start; + + printk("%.*s", (end - start) + 1, buf); +} + +void printk_nmi_complete(void) +{ + struct nmi_seq_buf *s; + int len, cpu, i, last_i; + + /* + * Now that all the NMIs have triggered, we can dump out their + * back traces safely to the console. + */ + for_each_possible_cpu(cpu) { + s = &per_cpu(nmi_print_seq, cpu); + last_i = 0; + + len = seq_buf_used(&s->seq); + if (!len) + continue; + + /* Print line by line. */ + for (i = 0; i < len; i++) { + if (s->buffer[i] == '\n') { + print_seq_line(s, last_i, i); + last_i = i + 1; + } + } + /* Check if there was a partial line. */ + if (last_i < len) { + print_seq_line(s, last_i, len - 1); + pr_cont("\n"); + } + + /* Wipe out the buffer ready for the next time around. */ + seq_buf_clear(&s->seq); + } + + clear_bit(0, &nmi_print_flag); + smp_mb__after_atomic(); +} + +void printk_nmi_this_cpu_begin(void) +{ + /* + * Detect double-begins and report them. This code is unsafe (because + * it will print from NMI) but things are pretty badly damaged if the + * NMI re-enters and is somehow granted permission to use NMI printk, + * so how much worse can it get? Also since this code interferes with + * the operation of printk it is unlikely that any consequential + * failures will be able to log anything making this our last + * opportunity to tell anyone that something is wrong. + */ + if (this_cpu_read(nmi_print_saved_print_func)) { + this_cpu_write(printk_func, vprintk_default); + BUG(); + } + + this_cpu_write(nmi_print_saved_print_func, this_cpu_read(printk_func)); + this_cpu_write(printk_func, nmi_vprintk); +} + +void printk_nmi_this_cpu_end(void) +{ + this_cpu_write(printk_func, this_cpu_read(nmi_print_saved_print_func)); + this_cpu_write(nmi_print_saved_print_func, NULL); +} + +#endif /* CONFIG_PRINTK_NMI */ + /** * printk - print a kernel message * @fmt: format string
Currently there is a quite a pile of code sitting in arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. The code is inaccessible to backtrace implementations for other architectures, which is a shame because they would probably like to be safe too. Copy this code into printk. We'll port the x86 NMI backtrace to it in a later patch. Incidentally, technically I think it might be safe to call printk_nmi_prepare() from NMI, providing care were taken to honour the return code. printk_nmi_complete() cannot be called from NMI but could be scheduled using irq_work_queue(). However honouring the return code means sometimes it is impossible to get the message out so in most cases I'd say using this code in such a way should probably attract sympathy and/or derision rather than admiration. Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org> Cc: Steven Rostedt <rostedt@goodmis.org> --- include/linux/printk.h | 18 ++++++ init/Kconfig | 3 + kernel/printk/printk.c | 149 +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 170 insertions(+)