cregit-Linux how code gets into the kernel

Release 4.15 kernel/printk/printk_safe.c

Directory: kernel/printk
 * 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
 * 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 <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>

#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.

static int printk_safe_irq_ready __read_mostly;

                                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_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);

/* Get flushed in a more safe context. */

static void queue_flush_work(struct printk_safe_seq_buf *s) { if (printk_safe_irq_ready) irq_work_queue(&s->work); }


Sergey Senozhatsky23100.00%1100.00%

/* * 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; 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(); add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args); 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); }


Sergey Senozhatsky2496.00%375.00%
Nicolas Iooss14.00%125.00%

/* 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; }


Petr Mladek17497.21%375.00%
Sergey Senozhatsky52.79%125.00%

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); }


Sergey Senozhatsky35100.00%1100.00%

/* * 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) { static raw_spinlock_t read_lock = __RAW_SPIN_LOCK_INITIALIZER(read_lock); 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(&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(&read_lock, flags); }


Petr Mladek15286.36%240.00%
Sergey Senozhatsky2413.64%360.00%

/** * 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); } }


Petr Mladek2553.19%133.33%
Sergey Senozhatsky2246.81%266.67%

/** * 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 (in_nmi() && raw_spin_is_locked(&logbuf_lock)) { if (num_online_cpus() > 1) return; debug_locks_off(); raw_spin_lock_init(&logbuf_lock); } printk_safe_flush(); }


Petr Mladek3995.12%150.00%
Sergey Senozhatsky24.88%150.00%

#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 printk_nmi_enter(void) { /* * The size of the extra per-CPU buffer is limited. Use it only when * the main one is locked. If this CPU is not in the safe context, * the lock must be taken on another CPU and we could wait for it. */ if ((this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) && raw_spin_is_locked(&logbuf_lock)) { this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK); } else { this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK); } }


Petr Mladek3068.18%150.00%
Sergey Senozhatsky1431.82%150.00%

void printk_nmi_exit(void) { this_cpu_and(printk_context, ~(PRINTK_NMI_CONTEXT_MASK | PRINTK_NMI_DEFERRED_CONTEXT_MASK)); }


Sergey Senozhatsky1578.95%150.00%
Petr Mladek421.05%150.00%

#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); }


Sergey Senozhatsky12100.00%1100.00%

/* Can be preempted by NMI. */
void __printk_safe_exit(void) { this_cpu_dec(printk_context); }


Sergey Senozhatsky12100.00%1100.00%

__printf(1, 0)
int vprintk_func(const char *fmt, va_list args) { /* 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); /* * Use the main logbuf when logbuf_lock is available in NMI. * But avoid calling console drivers that might have their own locks. */ if (this_cpu_read(printk_context) & PRINTK_NMI_DEFERRED_CONTEXT_MASK) return vprintk_deferred(fmt, args); /* No obstacles. */ return vprintk_default(fmt, args); }


Sergey Senozhatsky5572.37%150.00%
Petr Mladek2127.63%150.00%

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 } /* * In the highly unlikely event that a NMI were to trigger at * this moment. Make sure IRQ work is set up before this * variable is set. */ barrier(); printk_safe_irq_ready = 1; /* Flush pending messages that did not have scheduled IRQ works. */ printk_safe_flush(); }


Petr Mladek4354.43%125.00%
Sergey Senozhatsky3443.04%250.00%
Steven Rostedt22.53%125.00%

Overall Contributors

Petr Mladek64758.24%642.86%
Sergey Senozhatsky40136.09%535.71%
Nicolas Iooss595.31%17.14%
Steven Rostedt30.27%17.14%
Baoquan He10.09%17.14%
Directory: kernel/printk
Information contained on this website is for historical information purposes only and does not indicate or represent copyright ownership.
Created with cregit.