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We have observed that arc_release() can be called concurrently with a l2arc in-flight write. Also, we have observed that arc_hdr_destroy() can be called from arc_write_done() for a zio with ZIO_FLAG_IO_REWRITE flag in similar circumstances. Previously the l2arc headers would be freed while leaking their associated compression buffers. Now the buffers are placed on l2arc_free_on_write list for delayed freeing. This is similar to what was already done to arc buffers that were supposed to be freed concurrently with in-flight writes of those buffers. In addition to fixing the discovered leaks this change also adds some protective code to assert that a compression buffer associated with a l2arc header is never leaked. A new kstat l2_cdata_free_on_write is added. It keeps a count of delayed compression buffer frees which previously would have been leaks. Tested by: Vitalij Satanivskij <satan@ukr.net> et al Requested by: many MFC after: 2 weeks Sponsored by: HybridCluster / ClusterHQ
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