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RDMAVT: Fix synchronization around percpu_ref
rvt_mregion uses percpu_ref for reference counting and RCU to protect accesses from lkey_table. When a rvt_mregion needs to be freed, it first gets unregistered from lkey_table and then rvt_check_refs() is called to wait for in-flight usages before the rvt_mregion is freed. rvt_check_refs() seems to have a couple issues. * It has a fast exit path which tests percpu_ref_is_zero(). However, a percpu_ref reading zero doesn't mean that the object can be released. In fact, the ->release() callback might not even have started executing yet. Proceeding with freeing can lead to use-after-free. * lkey_table is RCU protected but there is no RCU grace period in the free path. percpu_ref uses RCU internally but it's sched-RCU whose grace periods are different from regular RCU. Also, it generally isn't a good idea to depend on internal behaviors like this. To address the above issues, this patch removes the fast exit and adds an explicit synchronize_rcu(). Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Dennis Dalessandro <dennis.dalessandro@intel.com> Cc: Mike Marciniszyn <mike.marciniszyn@intel.com> Cc: linux-rdma@vger.kernel.org Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
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@ -489,11 +489,13 @@ static int rvt_check_refs(struct rvt_mregion *mr, const char *t)
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unsigned long timeout;
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struct rvt_dev_info *rdi = ib_to_rvt(mr->pd->device);
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if (percpu_ref_is_zero(&mr->refcount))
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return 0;
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/* avoid dma mr */
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if (mr->lkey)
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if (mr->lkey) {
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/* avoid dma mr */
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rvt_dereg_clean_qps(mr);
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/* @mr was indexed on rcu protected @lkey_table */
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synchronize_rcu();
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}
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timeout = wait_for_completion_timeout(&mr->comp, 5 * HZ);
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if (!timeout) {
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rvt_pr_err(rdi,
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