On Fri, Jul 10, 2020 at 03:58:49PM -0700, Paul E. McKenney wrote: > Sadly, the code creating the 32-bit VDSO became unhappy: > > ------------------------------------------------------------------------ > > In file included from ././include/linux/compiler_types.h:59:0, > from <command-line>:0: > ./include/linux/smp.h:40:26: error: requested alignment is not a positive > power of 2 > __aligned(sizeof(struct __call_single_data)); > ^ > ./include/linux/compiler_attributes.h:57:68: note: in definition of macro > ‘__aligned’ > #define __aligned(x) __attribute__((__aligned__(x))) > ^ > scripts/Makefile.build:280: recipe for target > 'arch/x86/entry/vdso/vdso32/vclock_gettime.o' failed > make[3]: *** [arch/x86/entry/vdso/vdso32/vclock_gettime.o] Error 1 >
*groan*, we have BUILD_VDSO* for that, but yeah, not pretty. > Thoughts? The biggest one is C++ comments and excessively long lines but let me go stare at more detail :-) > ------------------------------------------------------------------------ > > commit aba04e2ce05fbc6bb90e6fa238c22e491f6b95e3 > Author: Paul E. McKenney <paul...@kernel.org> > Date: Tue Jun 30 13:22:54 2020 -0700 > > kernel/smp: Provide CSD lock timeout diagnostics > > This commit causes csd_lock_wait() to emit diagnostics when a CPU > fails to respond quickly enough to one of the smp_call_function() > family of function calls. These diagnostics are enabled by a new > CSD_LOCK_WAIT_DEBUG Kconfig option that depends on DEBUG_KERNEL. > > This commit was inspired by an earlier patch by Josef Bacik. > > [ paulmck: Fix for syzbot+0f719294463916a3f...@syzkaller.appspotmail.com ] > [ paulmck: Fix KASAN use-after-free issue reported by Qian Cai. ] > [ paulmck: Fix botched nr_cpu_ids comparison per Dan Carpenter. ] > [ paulmck: More #ifdef per Peter Zijlstra. ] > [ paulmck: Print delays in ns rather than ms per Peter Zijlstra. ] > [ paulmck: Use more CONFIG_CSD_LOCK_WAIT_DEBUG per Peter Zijlstra. ] > Link: https://lore.kernel.org/lkml/00000000000042f21905a991e...@google.com > Link: https://lore.kernel.org/lkml/0000000000002ef21705a9933...@google.com > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Ingo Molnar <mi...@kernel.org> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Sebastian Andrzej Siewior <bige...@linutronix.de> > Signed-off-by: Paul E. McKenney <paul...@kernel.org> > > diff --git a/kernel/smp.c b/kernel/smp.c > index 148d991..e1d8e52 100644 > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -20,6 +20,9 @@ > #include <linux/sched.h> > #include <linux/sched/idle.h> > #include <linux/hypervisor.h> > +#include <linux/sched/clock.h> > +#include <linux/nmi.h> > +#include <linux/sched/debug.h> > > #include "smpboot.h" > #include "sched/smp.h" > @@ -34,6 +37,12 @@ struct call_function_data { > > static DEFINE_PER_CPU_ALIGNED(struct call_function_data, cfd_data); > > +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG > +static DEFINE_PER_CPU(call_single_data_t *, cur_csd); > +static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func); > +static DEFINE_PER_CPU(void *, cur_csd_info); > +#endif None of these are used before the other #ifdef block below, perhaps merge them into one? > static DEFINE_PER_CPU_SHARED_ALIGNED(struct llist_head, call_single_queue); > > static void flush_smp_call_function_queue(bool warn_cpu_offline); > @@ -96,6 +105,103 @@ void __init call_function_init(void) > smpcfd_prepare_cpu(smp_processor_id()); > } > > +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG > + > +#define CSD_LOCK_TIMEOUT (5 * 1000ULL * 1000ULL * 1000ULL) /* Nanoseconds. */ NSEC_PER_SEC, that also instantly obviates the comment > +atomic_t csd_bug_count = ATOMIC_INIT(0); > + > +/* Record current CSD work for current CPU, NULL to erase. */ > +static void csd_lock_record(call_single_data_t *csd) > +{ > + if (!csd) { > + smp_mb(); // NULL cur_csd after unlock. > + __this_cpu_write(cur_csd, NULL); > + return; > + } > + __this_cpu_write(cur_csd, csd); > + __this_cpu_write(cur_csd_func, csd->func); > + __this_cpu_write(cur_csd_info, csd->info); > + smp_mb(); // Update cur_csd before function call. > + // Or before unlock, as the case may be. > +} > + > +static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd) > +{ > + unsigned int csd_type; > + > + csd_type = CSD_TYPE(csd); > + if (csd_type == CSD_TYPE_ASYNC || csd_type == CSD_TYPE_SYNC) > + return csd->dst; // Other CSD_TYPE_ values might not have ->dst. > + return -1; > +} > + > +/* > + * Complain if too much time spent waiting. Note that only > + * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU, > + * so waiting on other types gets much less information. > + */ > +static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd, > u64 ts0, u64 *ts1, int *bug_id) > +{ > + int cpu = -1; > + call_single_data_t *cpu_cur_csd; > + bool firsttime; > + unsigned int flags = READ_ONCE(csd->flags); > + u64 ts2, ts_delta; x-mas tree ? > + > + if (!(flags & CSD_FLAG_LOCK)) { > + if (!unlikely(*bug_id)) > + return true; > + cpu = csd_lock_wait_getcpu(csd); > + if (cpu >= 0) > + pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, > CPU#%02d released the lock after all. Phew!\n", *bug_id, > raw_smp_processor_id(), cpu); > + else > + pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, > the lock was released after all. Phew!\n", *bug_id, raw_smp_processor_id()); Excessively long line. Why not a single: pr_alert("csd: CSD lock (%d) got unstuck on CPU%d, CPU%d released the lock.\n", *bug_id, raw_smp_processor_id(), cpu); Yes, it'll print CPU-1, but given you have to more or less know this code intimately to make sense of this stuff, that seems like a fair trade-off. > + return true; > + } > + > + ts2 = sched_clock(); > + ts_delta = ts2 - *ts1; > + if (likely(ts_delta <= CSD_LOCK_TIMEOUT)) { > + cpu_relax(); > + return false; > + } > + > + firsttime = !*bug_id; > + if (firsttime) > + *bug_id = atomic_inc_return(&csd_bug_count); > + cpu = csd_lock_wait_getcpu(csd); > + smp_mb(); // No stale cur_csd values! > + if (WARN_ONCE(cpu < 0 || cpu >= nr_cpu_ids, "%s: cpu = %d\n", __func__, > cpu)) > + cpu_cur_csd = READ_ONCE(per_cpu(cur_csd, 0)); > + else > + cpu_cur_csd = READ_ONCE(per_cpu(cur_csd, cpu)); This is a potential user-after-free, func() may free the csd when async. Although I don't believe anybody does so. > + smp_mb(); // No refetching cur_csd values! The READ_ONCE() already ensures things aren't re-fetched, don't see why we'd need smp_mb() for that. The below is unreadable... espescially when smashed together without whitespace, also due to the stupid long lines it wraps and becomes a mangled mess of letters. > +#define CSD_FORMAT_PREFIX "csd: %s non-responsive CSD lock (#%d) on CPU#%d, > waiting %llu ns for CPU#%02d %pS(%ps), currently" > +#define CSD_ARGS_PREFIX firsttime ? "Detected" : "Continued", *bug_id, > raw_smp_processor_id(), \ > + ts2 - ts0, cpu, csd->func, csd->info > + if (cpu_cur_csd && csd != cpu_cur_csd) > + pr_alert(CSD_FORMAT_PREFIX " handling prior %pS(%ps) > request.\n", > + CSD_ARGS_PREFIX, cpu_cur_csd->func, cpu_cur_csd->info); > + else > + pr_alert(CSD_FORMAT_PREFIX " %s.\n", CSD_ARGS_PREFIX, > + !cpu_cur_csd ? "unresponsive" : "handling this > request"); definitely missing {} > +#undef CSD_FORMAT_PREFIX > +#undef CSD_ARGS_PREFIX Aside from it being unreadable, I've also completely lost the plot on what it's doing. > + if (cpu >= 0) { > + if (!trigger_single_cpu_backtrace(cpu)) > + dump_cpu_task(cpu); > + if (!cpu_cur_csd) { > + pr_alert("csd: Re-sending CSD lock (#%d) IPI from > CPU#%02d to CPU#%02d\n", *bug_id, raw_smp_processor_id(), cpu); > + arch_send_call_function_single_ipi(cpu); > + } > + } > + dump_stack(); > + *ts1 = ts2; I was expecting: *ts1 += CSD_LOCK_TIMEOUT; Not that it matters a great deal.. > + cpu_relax(); > + > + return false; > +} > + > /* > * csd_lock/csd_unlock used to serialize access to per-cpu csd resources > * > @@ -105,8 +211,26 @@ void __init call_function_init(void) > */ > static __always_inline void csd_lock_wait(call_single_data_t *csd) > { > + int bug_id = 0; > + u64 ts0, ts1; > + > + ts1 = ts0 = sched_clock(); > + for (;;) > + if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id)) > + break; This is lacking {}, also I think you can stick (at least) the cpu_relax() in this loop. for (;;) { if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id)) break; cpu_relax(); } And possible even the wait loop itself: for (;;) { if (csd_got_unlocked(csd, ...)) break; now = sched_clock(); if (now - ts1 > CSD_LOCK_TIMEOUT) { csd_held_too_long(csd, ts0, ts1, now, ...); ts1 += CSD_LOCK_TIMEOUT; } cpu_relax(); } that also allows breaking up that unreadable monster a bit. > + smp_acquire__after_ctrl_dep(); > +}
