On Mon 2019-02-11 13:50:35, Andrea Parri wrote: > Hi Michael, > > > On Thu, Feb 07, 2019 at 11:46:29PM +1100, Michael Ellerman wrote: > > Arch code can set a "dump stack arch description string" which is > > displayed with oops output to describe the hardware platform. > > > > It is useful to initialise this as early as possible, so that an early > > oops will have the hardware description. > > > > However in practice we discover the hardware platform in stages, so it > > would be useful to be able to incrementally fill in the hardware > > description as we discover it. > > > > This patch adds that ability, by creating dump_stack_add_arch_desc(). > > > > If there is no existing string it behaves exactly like > > dump_stack_set_arch_desc(). However if there is an existing string it > > appends to it, with a leading space. > > > > This makes it easy to call it multiple times from different parts of the > > code and get a reasonable looking result. > > > > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > > --- > > include/linux/printk.h | 5 ++++ > > lib/dump_stack.c | 58 ++++++++++++++++++++++++++++++++++++++++++ > > 2 files changed, 63 insertions(+) > > > > v3: No change, just widened Cc list. > > > > v2: Add a smp_wmb() and comment. > > > > v1 is here for reference https://lore.kernel.org/lkml/1430824337-15339-1-git-send-email-mpe@xxxxxxxxxxxxxx/ > > > > I'll take this series via the powerpc tree if no one minds? > > > > > > diff --git a/include/linux/printk.h b/include/linux/printk.h > > index 77740a506ebb..d5fb4f960271 100644 > > --- a/include/linux/printk.h > > +++ b/include/linux/printk.h > > @@ -198,6 +198,7 @@ u32 log_buf_len_get(void); > > void log_buf_vmcoreinfo_setup(void); > > void __init setup_log_buf(int early); > > __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...); > > +__printf(1, 2) void dump_stack_add_arch_desc(const char *fmt, ...); > > void dump_stack_print_info(const char *log_lvl); > > void show_regs_print_info(const char *log_lvl); > > extern asmlinkage void dump_stack(void) __cold; > > @@ -256,6 +257,10 @@ static inline __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...) > > { > > } > > > > +static inline __printf(1, 2) void dump_stack_add_arch_desc(const char *fmt, ...) > > +{ > > +} > > + > > static inline void dump_stack_print_info(const char *log_lvl) > > { > > } > > diff --git a/lib/dump_stack.c b/lib/dump_stack.c > > index 5cff72f18c4a..69b710ff92b5 100644 > > --- a/lib/dump_stack.c > > +++ b/lib/dump_stack.c > > @@ -35,6 +35,64 @@ void __init dump_stack_set_arch_desc(const char *fmt, ...) > > va_end(args); > > } > > > > +/** > > + * dump_stack_add_arch_desc - add arch-specific info to show with task dumps > > + * @fmt: printf-style format string > > + * @...: arguments for the format string > > + * > > + * See dump_stack_set_arch_desc() for why you'd want to use this. > > + * > > + * This version adds to any existing string already created with either > > + * dump_stack_set_arch_desc() or dump_stack_add_arch_desc(). If there is an > > + * existing string a space will be prepended to the passed string. > > + */ > > +void __init dump_stack_add_arch_desc(const char *fmt, ...) > > +{ > > + va_list args; > > + int pos, len; > > + char *p; > > + > > + /* > > + * If there's an existing string we snprintf() past the end of it, and > > + * then turn the terminating NULL of the existing string into a space > > + * to create one string separated by a space. > > + * > > + * If there's no existing string we just snprintf() to the buffer, like > > + * dump_stack_set_arch_desc(), but without calling it because we'd need > > + * a varargs version. > > + */ > > + len = strnlen(dump_stack_arch_desc_str, sizeof(dump_stack_arch_desc_str)); > > + pos = len; > > + > > + if (len) > > + pos++; > > + > > + if (pos >= sizeof(dump_stack_arch_desc_str)) > > + return; /* Ran out of space */ > > + > > + p = &dump_stack_arch_desc_str[pos]; > > + > > + va_start(args, fmt); > > + vsnprintf(p, sizeof(dump_stack_arch_desc_str) - pos, fmt, args); > > + va_end(args); > > + > > + if (len) { > > + /* > > + * Order the stores above in vsnprintf() vs the store of the > > + * space below which joins the two strings. Note this doesn't > > + * make the code truly race free because there is no barrier on > > + * the read side. ie. Another CPU might load the uninitialised > > + * tail of the buffer first and then the space below (rather > > + * than the NULL that was there previously), and so print the > > + * uninitialised tail. But the whole string lives in BSS so in > > + * practice it should just see NULLs. > > The comment doesn't say _why_ we need to order these stores: IOW, what > will or can go wrong without this order? This isn't clear to me. > > Another good practice when adding smp_*-constructs (as discussed, e.g., > at KS'18) is to indicate the matching construct/synch. mechanism. Yes, one barrier without a counter-part is suspicious. If the parallel access is really needed then we could define the current length as atomic_t and use: + atomic_cmpxchg() to reserve the space for the string + %*s to limit the printed length In the worst case, we would print an incomplete string. See below for a sample code. BTW: There are very few users of dump_stack_set_arch_desc(). I would use dump_stack_add_arch_desc() everywhere to keep it simple and have a reasonable semantic. This is what I mean (only compile tested): diff --git a/lib/dump_stack.c b/lib/dump_stack.c index 5cff72f18c4a..311dd20cc6a7 100644 --- a/lib/dump_stack.c +++ b/lib/dump_stack.c @@ -14,9 +14,10 @@ #include <linux/utsname.h> static char dump_stack_arch_desc_str[128]; +static atomic_t arch_desc_str_len; /** - * dump_stack_set_arch_desc - set arch-specific str to show with task dumps + * dump_stack_set_arch_desc - add arch-specific str to show with task dumps * @fmt: printf-style format string * @...: arguments for the format string * @@ -25,13 +26,32 @@ static char dump_stack_arch_desc_str[128]; * arch wants to make use of such an ID string, it should initialize this * as soon as possible during boot. */ -void __init dump_stack_set_arch_desc(const char *fmt, ...) +void __init dump_stack_add_arch_desc(const char *fmt, ...) { - va_list args; + va_list args, args2; + int len, cur_len, old_len; va_start(args, fmt); - vsnprintf(dump_stack_arch_desc_str, sizeof(dump_stack_arch_desc_str), + + va_copy(args2, args); + len = vsnprintf(NULL, sizeof(dump_stack_arch_desc_str), + fmt, args2); + va_end(args2); + +try_again: + cur_len = atomic_read(&arch_desc_str_len); + if (cur_len + len > sizeof(dump_stack_arch_desc_str)) + goto out; + + old_len = atomic_cmpxchg(&arch_desc_str_len, + cur_len, cur_len + len); + if (old_len != cur_len) + goto try_again; + + vsnprintf(dump_stack_arch_desc_str + old_len, + sizeof(dump_stack_arch_desc_str) - old_len, fmt, args); +out: va_end(args); } @@ -44,6 +64,8 @@ void __init dump_stack_set_arch_desc(const char *fmt, ...) */ void dump_stack_print_info(const char *log_lvl) { + int len; + printk("%sCPU: %d PID: %d Comm: %.20s %s%s %s %.*s\n", log_lvl, raw_smp_processor_id(), current->pid, current->comm, kexec_crash_loaded() ? "Kdump: loaded " : "", @@ -52,9 +74,11 @@ void dump_stack_print_info(const char *log_lvl) (int)strcspn(init_utsname()->version, " "), init_utsname()->version); - if (dump_stack_arch_desc_str[0] != '\0') - printk("%sHardware name: %s\n", - log_lvl, dump_stack_arch_desc_str); + len = atomic_read(&arch_desc_str_len); + if (len) { + printk("%sHardware name: %*s\n", + log_lvl, len, dump_stack_arch_desc_str); + } print_worker_info(log_lvl, current); } Best Regards, Petr
