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![Peter Zijlstra](/assets/img/avatar_default.png)
Currently the percpu-rwsem switches to (global) atomic ops while a writer is waiting; which could be quite a while and slows down releasing the readers. This patch cures this problem by ordering the reader-state vs reader-count (see the comments in __percpu_down_read() and percpu_down_write()). This changes a global atomic op into a full memory barrier, which doesn't have the global cacheline contention. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. 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: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org [ Fixed modular build. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
193 lines
5.1 KiB
C
193 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/wait.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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init_waitqueue_head(&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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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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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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