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![Davidlohr Bueso](/assets/img/avatar_default.png)
The use of any kind of wait queue is an overkill for pcpu-rwsems. While one option would be to use the less heavy simple (swait) flavor, this is still too much for what pcpu-rwsems needs. For one, we do not care about any sort of queuing in that the only (rare) time writers (and readers, for that matter) are queued is when trying to acquire the regular contended rw_sem. There cannot be any further queuing as writers are serialized by the rw_sem in the first place. Given that percpu_down_write() must not be called after exit_notify(), we can replace the bulky waitqueue with rcuwait such that a writer can wait for its turn to take the lock. As such, we can avoid the queue handling and locking overhead. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: dave@stgolabs.net Link: http://lkml.kernel.org/r/1484148146-14210-3-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
192 lines
5.1 KiB
C
192 lines
5.1 KiB
C
#include <linux/atomic.h>
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#include <linux/rwsem.h>
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#include <linux/percpu.h>
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#include <linux/lockdep.h>
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#include <linux/percpu-rwsem.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/errno.h>
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int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
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const char *name, struct lock_class_key *rwsem_key)
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{
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sem->read_count = alloc_percpu(int);
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if (unlikely(!sem->read_count))
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return -ENOMEM;
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/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
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rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
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__init_rwsem(&sem->rw_sem, name, rwsem_key);
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rcuwait_init(&sem->writer);
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sem->readers_block = 0;
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return 0;
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}
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EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
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void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
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{
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/*
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* XXX: temporary kludge. The error path in alloc_super()
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* assumes that percpu_free_rwsem() is safe after kzalloc().
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*/
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if (!sem->read_count)
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return;
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rcu_sync_dtor(&sem->rss);
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free_percpu(sem->read_count);
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sem->read_count = NULL; /* catch use after free bugs */
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}
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EXPORT_SYMBOL_GPL(percpu_free_rwsem);
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int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
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{
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/*
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* Due to having preemption disabled the decrement happens on
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* the same CPU as the increment, avoiding the
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* increment-on-one-CPU-and-decrement-on-another problem.
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*
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* If the reader misses the writer's assignment of readers_block, then
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* the writer is guaranteed to see the reader's increment.
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*
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* Conversely, any readers that increment their sem->read_count after
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* the writer looks are guaranteed to see the readers_block value,
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* which in turn means that they are guaranteed to immediately
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* decrement their sem->read_count, so that it doesn't matter that the
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* writer missed them.
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*/
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smp_mb(); /* A matches D */
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/*
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* If !readers_block the critical section starts here, matched by the
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* release in percpu_up_write().
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*/
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if (likely(!smp_load_acquire(&sem->readers_block)))
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return 1;
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/*
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* Per the above comment; we still have preemption disabled and
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* will thus decrement on the same CPU as we incremented.
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*/
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__percpu_up_read(sem);
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if (try)
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return 0;
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/*
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* We either call schedule() in the wait, or we'll fall through
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* and reschedule on the preempt_enable() in percpu_down_read().
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*/
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preempt_enable_no_resched();
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/*
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* Avoid lockdep for the down/up_read() we already have them.
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*/
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__down_read(&sem->rw_sem);
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this_cpu_inc(*sem->read_count);
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__up_read(&sem->rw_sem);
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preempt_disable();
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return 1;
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}
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EXPORT_SYMBOL_GPL(__percpu_down_read);
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void __percpu_up_read(struct percpu_rw_semaphore *sem)
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{
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smp_mb(); /* B matches C */
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/*
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* In other words, if they see our decrement (presumably to aggregate
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* zero, as that is the only time it matters) they will also see our
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* critical section.
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*/
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__this_cpu_dec(*sem->read_count);
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/* Prod writer to recheck readers_active */
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rcuwait_wake_up(&sem->writer);
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}
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EXPORT_SYMBOL_GPL(__percpu_up_read);
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#define per_cpu_sum(var) \
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({ \
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typeof(var) __sum = 0; \
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int cpu; \
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compiletime_assert_atomic_type(__sum); \
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for_each_possible_cpu(cpu) \
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__sum += per_cpu(var, cpu); \
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__sum; \
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})
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/*
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* Return true if the modular sum of the sem->read_count per-CPU variable is
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* zero. If this sum is zero, then it is stable due to the fact that if any
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* newly arriving readers increment a given counter, they will immediately
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* decrement that same counter.
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*/
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static bool readers_active_check(struct percpu_rw_semaphore *sem)
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{
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if (per_cpu_sum(*sem->read_count) != 0)
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return false;
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/*
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* If we observed the decrement; ensure we see the entire critical
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* section.
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*/
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smp_mb(); /* C matches B */
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return true;
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}
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void percpu_down_write(struct percpu_rw_semaphore *sem)
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{
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/* Notify readers to take the slow path. */
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rcu_sync_enter(&sem->rss);
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down_write(&sem->rw_sem);
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/*
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* Notify new readers to block; up until now, and thus throughout the
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* longish rcu_sync_enter() above, new readers could still come in.
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*/
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WRITE_ONCE(sem->readers_block, 1);
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smp_mb(); /* D matches A */
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/*
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* If they don't see our writer of readers_block, then we are
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* guaranteed to see their sem->read_count increment, and therefore
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* will wait for them.
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*/
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/* Wait for all now active readers to complete. */
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rcuwait_wait_event(&sem->writer, readers_active_check(sem));
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}
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EXPORT_SYMBOL_GPL(percpu_down_write);
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void percpu_up_write(struct percpu_rw_semaphore *sem)
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{
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/*
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* Signal the writer is done, no fast path yet.
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*
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* One reason that we cannot just immediately flip to readers_fast is
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* that new readers might fail to see the results of this writer's
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* critical section.
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*
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* Therefore we force it through the slow path which guarantees an
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* acquire and thereby guarantees the critical section's consistency.
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*/
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smp_store_release(&sem->readers_block, 0);
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/*
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* Release the write lock, this will allow readers back in the game.
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*/
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up_write(&sem->rw_sem);
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/*
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* Once this completes (at least one RCU-sched grace period hence) the
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* reader fast path will be available again. Safe to use outside the
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* exclusive write lock because its counting.
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*/
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rcu_sync_exit(&sem->rss);
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}
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EXPORT_SYMBOL_GPL(percpu_up_write);
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