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Change preempt_disable() etc. to use a recursive mutex.

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This is part of the userspace implementation of the kernel APIs for
bcachefs-tools. The previous implementation just provided a barrier, but
this isn't sufficient to make the associated percpu implementation safe.

Signed-off-by: Justin Husted <sigstop@gmail.com>
This commit is contained in:
Justin Husted 2019-10-09 19:27:22 -07:00 committed by Kent Overstreet
parent 2bc1166919
commit ff4d082246
2 changed files with 36 additions and 7 deletions
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15
include/linux/preempt.h
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@ -1,15 +1,16 @@
#ifndef __LINUX_PREEMPT_H
#define __LINUX_PREEMPT_H
#define preempt_disable() barrier()
#define sched_preempt_enable_no_resched() barrier()
#define preempt_enable_no_resched() barrier()
#define preempt_enable() barrier()
extern void preempt_disable(void);
extern void preempt_enable(void);
#define sched_preempt_enable_no_resched() preempt_enable()
#define preempt_enable_no_resched() preempt_enable()
#define preempt_check_resched() do { } while (0)
#define preempt_disable_notrace() barrier()
#define preempt_enable_no_resched_notrace() barrier()
#define preempt_enable_notrace() barrier()
#define preempt_disable_notrace() preempt_disable()
#define preempt_enable_no_resched_notrace() preempt_enable()
#define preempt_enable_notrace() preempt_enable()
#define preemptible() 0
#endif /* __LINUX_PREEMPT_H */

28
linux/preempt.c Normal file
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@ -0,0 +1,28 @@
#include <pthread.h>
#include "linux/preempt.h"
/*
* In userspace, pthreads are preemptible and can migrate CPUs at any time.
*
* In the kernel, preempt_disable() logic essentially guarantees that a marked
* critical section owns its CPU for the relevant block. This is necessary for
* various code paths, critically including the percpu system as it allows for
* non-atomic reads and writes to CPU-local data structures.
*
* The high performance userspace equivalent would be to use thread local
* storage to replace percpu data, but that would be complicated. It should be
* correct to instead guarantee mutual exclusion for the critical sections.
*/
static pthread_mutex_t preempt_lock = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
void preempt_disable(void)
{
pthread_mutex_lock(&preempt_lock);
}
void preempt_enable(void)
{
pthread_mutex_unlock(&preempt_lock);
}