/* * printk_safe.c - Safe printk for printk-deadlock-prone contexts * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version 2 * of the License, or (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, see . */ #include #include #include #include #include #include #include #include "internal.h" /* * printk() could not take logbuf_lock in NMI context. Instead, * it uses an alternative implementation that temporary stores * the strings into a per-CPU buffer. The content of the buffer * is later flushed into the main ring buffer via IRQ work. * * The alternative implementation is chosen transparently * by examinig current printk() context mask stored in @printk_context * per-CPU variable. * * The implementation allows to flush the strings also from another CPU. * There are situations when we want to make sure that all buffers * were handled or when IRQs are blocked. */ #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \ sizeof(atomic_t) - \ sizeof(atomic_t) - \ sizeof(struct irq_work)) struct printk_safe_seq_buf { atomic_t len; /* length of written data */ atomic_t message_lost; struct irq_work work; /* IRQ work that flushes the buffer */ unsigned char buffer[SAFE_LOG_BUF_LEN]; }; static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq); static DEFINE_PER_CPU(int, printk_context); static DEFINE_RAW_SPINLOCK(safe_read_lock); #ifdef CONFIG_PRINTK_NMI static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq); #endif /* Get flushed in a more safe context. */ static void queue_flush_work(struct printk_safe_seq_buf *s) { if (printk_percpu_data_ready()) irq_work_queue(&s->work); } /* * Add a message to per-CPU context-dependent buffer. NMI and printk-safe * have dedicated buffers, because otherwise printk-safe preempted by * NMI-printk would have overwritten the NMI messages. * * The messages are flushed from irq work (or from panic()), possibly, * from other CPU, concurrently with printk_safe_log_store(). Should this * happen, printk_safe_log_store() will notice the buffer->len mismatch * and repeat the write. */ static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s, const char *fmt, va_list args) { int add; size_t len; va_list ap; again: len = atomic_read(&s->len); /* The trailing '\0' is not counted into len. */ if (len >= sizeof(s->buffer) - 1) { atomic_inc(&s->message_lost); queue_flush_work(s); return 0; } /* * Make sure that all old data have been read before the buffer * was reset. This is not needed when we just append data. */ if (!len) smp_rmb(); va_copy(ap, args); add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap); va_end(ap); if (!add) return 0; /* * Do it once again if the buffer has been flushed in the meantime. * Note that atomic_cmpxchg() is an implicit memory barrier that * makes sure that the data were written before updating s->len. */ if (atomic_cmpxchg(&s->len, len, len + add) != len) goto again; queue_flush_work(s); return add; } static inline void printk_safe_flush_line(const char *text, int len) { /* * Avoid any console drivers calls from here, because we may be * in NMI or printk_safe context (when in panic). The messages * must go only into the ring buffer at this stage. Consoles will * get explicitly called later when a crashdump is not generated. */ printk_deferred("%.*s", len, text); } /* printk part of the temporary buffer line by line */ static int printk_safe_flush_buffer(const char *start, size_t len) { const char *c, *end; bool header; c = start; end = start + len; header = true; /* Print line by line. */ while (c < end) { if (*c == '\n') { printk_safe_flush_line(start, c - start + 1); start = ++c; header = true; continue; } /* Handle continuous lines or missing new line. */ if ((c + 1 < end) && printk_get_level(c)) { if (header) { c = printk_skip_level(c); continue; } printk_safe_flush_line(start, c - start); start = c++; header = true; continue; } header = false; c++; } /* Check if there was a partial line. Ignore pure header. */ if (start < end && !header) { static const char newline[] = KERN_CONT "\n"; printk_safe_flush_line(start, end - start); printk_safe_flush_line(newline, strlen(newline)); } return len; } static void report_message_lost(struct printk_safe_seq_buf *s) { int lost = atomic_xchg(&s->message_lost, 0); if (lost) printk_deferred("Lost %d message(s)!\n", lost); } /* * Flush data from the associated per-CPU buffer. The function * can be called either via IRQ work or independently. */ static void __printk_safe_flush(struct irq_work *work) { struct printk_safe_seq_buf *s = container_of(work, struct printk_safe_seq_buf, work); unsigned long flags; size_t len; int i; /* * The lock has two functions. First, one reader has to flush all * available message to make the lockless synchronization with * writers easier. Second, we do not want to mix messages from * different CPUs. This is especially important when printing * a backtrace. */ raw_spin_lock_irqsave(&safe_read_lock, flags); i = 0; more: len = atomic_read(&s->len); /* * This is just a paranoid check that nobody has manipulated * the buffer an unexpected way. If we printed something then * @len must only increase. Also it should never overflow the * buffer size. */ if ((i && i >= len) || len > sizeof(s->buffer)) { const char *msg = "printk_safe_flush: internal error\n"; printk_safe_flush_line(msg, strlen(msg)); len = 0; } if (!len) goto out; /* Someone else has already flushed the buffer. */ /* Make sure that data has been written up to the @len */ smp_rmb(); i += printk_safe_flush_buffer(s->buffer + i, len - i); /* * Check that nothing has got added in the meantime and truncate * the buffer. Note that atomic_cmpxchg() is an implicit memory * barrier that makes sure that the data were copied before * updating s->len. */ if (atomic_cmpxchg(&s->len, len, 0) != len) goto more; out: report_message_lost(s); raw_spin_unlock_irqrestore(&safe_read_lock, flags); } /** * printk_safe_flush - flush all per-cpu nmi buffers. * * The buffers are flushed automatically via IRQ work. This function * is useful only when someone wants to be sure that all buffers have * been flushed at some point. */ void printk_safe_flush(void) { int cpu; for_each_possible_cpu(cpu) { #ifdef CONFIG_PRINTK_NMI __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work); #endif __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work); } } /** * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system * goes down. * * Similar to printk_safe_flush() but it can be called even in NMI context when * the system goes down. It does the best effort to get NMI messages into * the main ring buffer. * * Note that it could try harder when there is only one CPU online. */ void printk_safe_flush_on_panic(void) { /* * Make sure that we could access the main ring buffer. * Do not risk a double release when more CPUs are up. */ if (raw_spin_is_locked(&logbuf_lock)) { if (num_online_cpus() > 1) return; debug_locks_off(); raw_spin_lock_init(&logbuf_lock); } if (raw_spin_is_locked(&safe_read_lock)) { if (num_online_cpus() > 1) return; debug_locks_off(); raw_spin_lock_init(&safe_read_lock); } printk_safe_flush(); } #ifdef CONFIG_PRINTK_NMI /* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only * one writer running. But the buffer might get flushed from another * CPU, so we need to be careful. */ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args) { struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq); return printk_safe_log_store(s, fmt, args); } void notrace printk_nmi_enter(void) { this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK); } void notrace printk_nmi_exit(void) { this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK); } /* * Marks a code that might produce many messages in NMI context * and the risk of losing them is more critical than eventual * reordering. * * It has effect only when called in NMI context. Then printk() * will try to store the messages into the main logbuf directly * and use the per-CPU buffers only as a fallback when the lock * is not available. */ void printk_nmi_direct_enter(void) { if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK) this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK); } void printk_nmi_direct_exit(void) { this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK); } #else static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args) { return 0; } #endif /* CONFIG_PRINTK_NMI */ /* * Lock-less printk(), to avoid deadlocks should the printk() recurse * into itself. It uses a per-CPU buffer to store the message, just like * NMI. */ static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args) { struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq); return printk_safe_log_store(s, fmt, args); } /* Can be preempted by NMI. */ void __printk_safe_enter(void) { this_cpu_inc(printk_context); } /* Can be preempted by NMI. */ void __printk_safe_exit(void) { this_cpu_dec(printk_context); } __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { /* * Try to use the main logbuf even in NMI. But avoid calling console * drivers that might have their own locks. */ if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) && raw_spin_trylock(&logbuf_lock)) { int len; len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args); raw_spin_unlock(&logbuf_lock); defer_console_output(); return len; } /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */ if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK) return vprintk_nmi(fmt, args); /* Use extra buffer to prevent a recursion deadlock in safe mode. */ if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) return vprintk_safe(fmt, args); /* No obstacles. */ return vprintk_default(fmt, args); } void __init printk_safe_init(void) { int cpu; for_each_possible_cpu(cpu) { struct printk_safe_seq_buf *s; s = &per_cpu(safe_print_seq, cpu); init_irq_work(&s->work, __printk_safe_flush); #ifdef CONFIG_PRINTK_NMI s = &per_cpu(nmi_print_seq, cpu); init_irq_work(&s->work, __printk_safe_flush); #endif } /* Flush pending messages that did not have scheduled IRQ works. */ printk_safe_flush(); }