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printk: rename cpulock functions

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Since the printk cpulock is CPU-reentrant and since it is used
in all contexts, its usage must be carefully considered and
most likely will require programming locklessly. To avoid
mistaking the printk cpulock as a typical lock, rename it to
cpu_sync. The main functions then become:

    printk_cpu_sync_get_irqsave(flags);
    printk_cpu_sync_put_irqrestore(flags);

Add extra notes of caution in the function description to help
developers understand the requirements for correct usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-2-john.ogness@linutronix.de
This commit is contained in:
John Ogness 2022-04-21 23:28:36 +02:06 committed by Petr Mladek
parent 3ef4ea3d84
commit faebd693c5
4 changed files with 73 additions and 60 deletions
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54
include/linux/printk.h
View File

@ -277,43 +277,55 @@ static inline void printk_trigger_flush(void)
#endif
#ifdef CONFIG_SMP
extern int __printk_cpu_trylock(void);
extern void __printk_wait_on_cpu_lock(void);
extern void __printk_cpu_unlock(void);
extern int __printk_cpu_sync_try_get(void);
extern void __printk_cpu_sync_wait(void);
extern void __printk_cpu_sync_put(void);
/**
* printk_cpu_lock_irqsave() - Acquire the printk cpu-reentrant spinning
* lock and disable interrupts.
* printk_cpu_sync_get_irqsave() - Acquire the printk cpu-reentrant spinning
* lock and disable interrupts.
* @flags: Stack-allocated storage for saving local interrupt state,
* to be passed to printk_cpu_unlock_irqrestore().
* to be passed to printk_cpu_sync_put_irqrestore().
*
* If the lock is owned by another CPU, spin until it becomes available.
* Interrupts are restored while spinning.
*
* CAUTION: This function must be used carefully. It does not behave like a
* typical lock. Here are important things to watch out for...
*
* * This function is reentrant on the same CPU. Therefore the calling
* code must not assume exclusive access to data if code accessing the
* data can run reentrant or within NMI context on the same CPU.
*
* * If there exists usage of this function from NMI context, it becomes
* unsafe to perform any type of locking or spinning to wait for other
* CPUs after calling this function from any context. This includes
* using spinlocks or any other busy-waiting synchronization methods.
*/
#define printk_cpu_lock_irqsave(flags) \
for (;;) { \
local_irq_save(flags); \
if (__printk_cpu_trylock()) \
break; \
local_irq_restore(flags); \
__printk_wait_on_cpu_lock(); \
#define printk_cpu_sync_get_irqsave(flags) \
for (;;) { \
local_irq_save(flags); \
if (__printk_cpu_sync_try_get()) \
break; \
local_irq_restore(flags); \
__printk_cpu_sync_wait(); \
}
/**
* printk_cpu_unlock_irqrestore() - Release the printk cpu-reentrant spinning
* lock and restore interrupts.
* @flags: Caller's saved interrupt state, from printk_cpu_lock_irqsave().
* printk_cpu_sync_put_irqrestore() - Release the printk cpu-reentrant spinning
* lock and restore interrupts.
* @flags: Caller's saved interrupt state, from printk_cpu_sync_get_irqsave().
*/
#define printk_cpu_unlock_irqrestore(flags) \
#define printk_cpu_sync_put_irqrestore(flags) \
do { \
__printk_cpu_unlock(); \
__printk_cpu_sync_put(); \
local_irq_restore(flags); \
} while (0) \
} while (0)
#else
#define printk_cpu_lock_irqsave(flags) ((void)flags)
#define printk_cpu_unlock_irqrestore(flags) ((void)flags)
#define printk_cpu_sync_get_irqsave(flags) ((void)flags)
#define printk_cpu_sync_put_irqrestore(flags) ((void)flags)
#endif /* CONFIG_SMP */

71
kernel/printk/printk.c
View File

@ -3667,26 +3667,26 @@ EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
#endif
#ifdef CONFIG_SMP
static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
static atomic_t printk_cpulock_nested = ATOMIC_INIT(0);
static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);
/**
* __printk_wait_on_cpu_lock() - Busy wait until the printk cpu-reentrant
* spinning lock is not owned by any CPU.
* __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
* spinning lock is not owned by any CPU.
*
* Context: Any context.
*/
void __printk_wait_on_cpu_lock(void)
void __printk_cpu_sync_wait(void)
{
do {
cpu_relax();
} while (atomic_read(&printk_cpulock_owner) != -1);
} while (atomic_read(&printk_cpu_sync_owner) != -1);
}
EXPORT_SYMBOL(__printk_wait_on_cpu_lock);
EXPORT_SYMBOL(__printk_cpu_sync_wait);
/**
* __printk_cpu_trylock() - Try to acquire the printk cpu-reentrant
* spinning lock.
* __printk_cpu_sync_try_get() - Try to acquire the printk cpu-reentrant
* spinning lock.
*
* If no processor has the lock, the calling processor takes the lock and
* becomes the owner. If the calling processor is already the owner of the
@ -3695,7 +3695,7 @@ EXPORT_SYMBOL(__printk_wait_on_cpu_lock);
* Context: Any context. Expects interrupts to be disabled.
* Return: 1 on success, otherwise 0.
*/
int __printk_cpu_trylock(void)
int __printk_cpu_sync_try_get(void)
{
int cpu;
int old;
@ -3705,79 +3705,80 @@ int __printk_cpu_trylock(void)
/*
* Guarantee loads and stores from this CPU when it is the lock owner
* are _not_ visible to the previous lock owner. This pairs with
* __printk_cpu_unlock:B.
* __printk_cpu_sync_put:B.
*
* Memory barrier involvement:
*
* If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, then
* __printk_cpu_unlock:A can never read from __printk_cpu_trylock:B.
* If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
* then __printk_cpu_sync_put:A can never read from
* __printk_cpu_sync_try_get:B.
*
* Relies on:
*
* RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
* RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
* of the previous CPU
* matching
* ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
* of this CPU
* ACQUIRE from __printk_cpu_sync_try_get:A to
* __printk_cpu_sync_try_get:B of this CPU
*/
old = atomic_cmpxchg_acquire(&printk_cpulock_owner, -1,
cpu); /* LMM(__printk_cpu_trylock:A) */
old = atomic_cmpxchg_acquire(&printk_cpu_sync_owner, -1,
cpu); /* LMM(__printk_cpu_sync_try_get:A) */
if (old == -1) {
/*
* This CPU is now the owner and begins loading/storing
* data: LMM(__printk_cpu_trylock:B)
* data: LMM(__printk_cpu_sync_try_get:B)
*/
return 1;
} else if (old == cpu) {
/* This CPU is already the owner. */
atomic_inc(&printk_cpulock_nested);
atomic_inc(&printk_cpu_sync_nested);
return 1;
}
return 0;
}
EXPORT_SYMBOL(__printk_cpu_trylock);
EXPORT_SYMBOL(__printk_cpu_sync_try_get);
/**
* __printk_cpu_unlock() - Release the printk cpu-reentrant spinning lock.
* __printk_cpu_sync_put() - Release the printk cpu-reentrant spinning lock.
*
* The calling processor must be the owner of the lock.
*
* Context: Any context. Expects interrupts to be disabled.
*/
void __printk_cpu_unlock(void)
void __printk_cpu_sync_put(void)
{
if (atomic_read(&printk_cpulock_nested)) {
atomic_dec(&printk_cpulock_nested);
if (atomic_read(&printk_cpu_sync_nested)) {
atomic_dec(&printk_cpu_sync_nested);
return;
}
/*
* This CPU is finished loading/storing data:
* LMM(__printk_cpu_unlock:A)
* LMM(__printk_cpu_sync_put:A)
*/
/*
* Guarantee loads and stores from this CPU when it was the
* lock owner are visible to the next lock owner. This pairs
* with __printk_cpu_trylock:A.
* with __printk_cpu_sync_try_get:A.
*
* Memory barrier involvement:
*
* If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B,
* then __printk_cpu_trylock:B reads from __printk_cpu_unlock:A.
* If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
* then __printk_cpu_sync_try_get:B reads from __printk_cpu_sync_put:A.
*
* Relies on:
*
* RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
* RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
* of this CPU
* matching
* ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
* of the next CPU
* ACQUIRE from __printk_cpu_sync_try_get:A to
* __printk_cpu_sync_try_get:B of the next CPU
*/
atomic_set_release(&printk_cpulock_owner,
-1); /* LMM(__printk_cpu_unlock:B) */
atomic_set_release(&printk_cpu_sync_owner,
-1); /* LMM(__printk_cpu_sync_put:B) */
}
EXPORT_SYMBOL(__printk_cpu_unlock);
EXPORT_SYMBOL(__printk_cpu_sync_put);
#endif /* CONFIG_SMP */

4
lib/dump_stack.c
View File

@ -102,9 +102,9 @@ asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
* Permit this cpu to perform nested stack dumps while serialising
* against other CPUs
*/
printk_cpu_lock_irqsave(flags);
printk_cpu_sync_get_irqsave(flags);
__dump_stack(log_lvl);
printk_cpu_unlock_irqrestore(flags);
printk_cpu_sync_put_irqrestore(flags);
}
EXPORT_SYMBOL(dump_stack_lvl);

4
lib/nmi_backtrace.c
View File

@ -99,7 +99,7 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
* Allow nested NMI backtraces while serializing
* against other CPUs.
*/
printk_cpu_lock_irqsave(flags);
printk_cpu_sync_get_irqsave(flags);
if (!READ_ONCE(backtrace_idle) && regs && cpu_in_idle(instruction_pointer(regs))) {
pr_warn("NMI backtrace for cpu %d skipped: idling at %pS\n",
cpu, (void *)instruction_pointer(regs));
@ -110,7 +110,7 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
else
dump_stack();
}
printk_cpu_unlock_irqrestore(flags);
printk_cpu_sync_put_irqrestore(flags);
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
return true;
}