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Message-ID: <87eet1j7x6.fsf@suse.de>
Date: Mon, 06 Apr 2020 11:11:01 +0200
From: Nicolai Stange <nstange@...e.de>
To: Nicolai Stange <nstange@...e.de>
Cc: Bart Van Assche <bvanassche@....org>,
Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
ming.lei@...hat.com, mhocko@...e.com, linux-block@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
Omar Sandoval <osandov@...com>,
Hannes Reinecke <hare@...e.com>,
Michal Hocko <mhocko@...nel.org>
Subject: Re: [RFC 3/3] block: avoid deferral of blk_release_queue() work
Nicolai Stange <nstange@...e.de> writes:
> Bart Van Assche <bvanassche@....org> 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.
Thanks,
Nicolai
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