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Avoid blocking in arc_reclaim_thread()

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As described in the comment above arc_reclaim_thread() it's critical
that the reclaim thread be careful about blocking.  Just like it must
never wait on a hash lock, it must never wait on a task which can in
turn wait on the CV in arc_get_data_buf().  This will deadlock, see
issue #3822 for full backtraces showing the problem.

To resolve this issue arc_kmem_reap_now() has been updated to use the
asynchronous arc prune function.  This means that arc_prune_async()
may now be called while there are still outstanding arc_prune_tasks.
However, this isn't a problem because arc_prune_async() already
keeps a reference count preventing multiple outstanding tasks per
registered consumer.  Functionally, this behavior is the same as
the counterpart illumos function dnlc_reduce_cache().

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #3808
Issue #3834
Issue #3822
This commit is contained in:
Brian Behlendorf 2015-09-23 15:59:04 -07:00
parent 04870568e6
commit ef5b2e1048
1 changed files with 4 additions and 11 deletions
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15
module/zfs/arc.c
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@ -2685,8 +2685,8 @@ arc_flush_state(arc_state_t *state, uint64_t spa, arc_buf_contents_t type,
}
/*
* Helper function for arc_prune() it is responsible for safely handling
* the execution of a registered arc_prune_func_t.
* Helper function for arc_prune_async() it is responsible for safely
* handling the execution of a registered arc_prune_func_t.
*/
static void
arc_prune_task(void *ptr)
@ -2711,7 +2711,7 @@ arc_prune_task(void *ptr)
* honor the arc_meta_limit and reclaim otherwise pinned ARC buffers. This
* is analogous to dnlc_reduce_cache() but more generic.
*
* This operation is performed asyncronously so it may be safely called
* This operation is performed asynchronously so it may be safely called
* in the context of the arc_reclaim_thread(). A reference is taken here
* for each registered arc_prune_t and the arc_prune_task() is responsible
* for releasing it once the registered arc_prune_func_t has completed.
@ -2736,13 +2736,6 @@ arc_prune_async(int64_t adjust)
mutex_exit(&arc_prune_mtx);
}
static void
arc_prune(int64_t adjust)
{
arc_prune_async(adjust);
taskq_wait_outstanding(arc_prune_taskq, 0);
}
/*
* Evict the specified number of bytes from the state specified,
* restricting eviction to the spa and type given. This function
@ -3376,7 +3369,7 @@ arc_kmem_reap_now(void)
* We are exceeding our meta-data cache limit.
* Prune some entries to release holds on meta-data.
*/
arc_prune(zfs_arc_meta_prune);
arc_prune_async(zfs_arc_meta_prune);
}
for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {