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SubjectRe: [RFC 3/3] block: avoid deferral of blk_release_queue() work
Nicolai Stange <> writes:

> Bart Van Assche <> writes:
>> The description of this patch mentions a single blk_release_queue() call
>> that happened in the past from a context from which sleeping is not
>> allowed and from which sleeping is allowed today. Have all other
>> blk_release_queue() / blk_put_queue() calls been verified to see whether
>> none of these happens from a context from which sleeping is not allowed?
> I've just done this today and found the following potentially
> problematic call paths to blk_put_queue().
> 1.) mem_cgroup_throttle_swaprate() takes a spinlock and
> calls blkcg_schedule_throttle()->blk_put_queue().
> Also note that AFAICS mem_cgroup_try_charge_delay() can be called
> with GFP_ATOMIC.
> 2.) scsi_unblock_requests() gets called from a lot of drivers and
> invoke blk_put_queue() through
> scsi_unblock_requests() -> scsi_run_host_queues() ->
> scsi_starved_list_run() -> blk_put_queue().
> Most call sites are fine, the ones which are not are:
> a.) pmcraid_complete_ioa_reset(). This gets assigned
> to struct pmcraid_cmd's ->cmd_done and later invoked
> under a spinlock.
> b.) qla82xx_fw_dump() and qla8044_fw_dump().
> These can potentially block w/o this patch already,
> because both invoke qla2x00_wait_for_chip_reset().
> However, they can get called from IRQ context. For example,
> qla82xx_intr_handler(), qla82xx_msix_default() and
> qla82xx_poll() call qla2x00_async_event(), which calls
> ->fw_dump().
> The aforementioned functions can also reach ->fw_dump() through
> qla24xx_process_response_queue()->qlt_handle_abts_recv()->qlt_response_pkt_all_vps()
> ->qlt_response_pkt()->qlt_handle_abts_completion()->qlt_chk_unresolv_exchg()
> -> ->fw_dump().
> But I'd consider this a problem with the driver -- either
> ->fw_dump() can sleep and must not be called from IRQ context
> or they must not invoke qla2x00_wait_for_hba_ready().
> (I can share the full analysis, but it's lengthy and contains nothing
> interesting except for what is listed above).
> One final note though: If I'm not mistaken, then the final
> blk_put_queue() can in principle block even today, simply by virtue of
> the kernfs operations invoked through
> kobject_put()->kobject_release()->kobject_cleanup()->kobject_del()
> ->sysfs_remove_dir()->kernfs_remove()->mutex_lock()?\

That's wrong, I missed kobject_del() invocation issued from
blk_unregister_queue(). Thus, blk_put_queue() in its current
implementation won't ever block.



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