From mboxrd@z Thu Jan  1 00:00:00 1970
From: jmoyer@redhat.com (Jeff Moyer)
Date: Tue, 24 Nov 2015 11:34:22 -0500
Subject: [PATCH 04/47] block: provide a new BLK_EH_QUIESCED timeout return
 value
In-Reply-To: <20151124155609.GA6251@lst.de> (Christoph Hellwig's message of
 "Tue, 24 Nov 2015 16:56:09 +0100")
References: <1448037342-18384-1-git-send-email-hch@lst.de>
 <1448037342-18384-5-git-send-email-hch@lst.de>
 <x49mvu32zrw.fsf@segfault.boston.devel.redhat.com>
 <20151124154019.GB5939@lst.de>
 <x491tbf2y6v.fsf@segfault.boston.devel.redhat.com>
 <20151124155609.GA6251@lst.de>
Message-ID: <x49wpt71hm9.fsf@segfault.boston.devel.redhat.com>

Christoph Hellwig <hch at lst.de> writes:

> We always set REQ_ATOM_COMPLETE before calling into ->timeout and thus
> even having a chance to REQ_ATOM_QUIESCED.  Maybe we're talking past
> each other, so if it feels like I'm off track try to explain the
> race in a bit more detail.

Sure.

CPU 0 executes the following:

blk_rq_check_expired():
        if (!blk_mark_rq_complete(rq))  // this succeeds, so we call blk_rq_timed_out
                blk_rq_timed_out(rq);
blk_rq_timed_out():
        if (q->rq_timed_out_fn)
                ret = q->rq_timed_out_fn(req);
        switch (ret) {  // QUIESCED is returned
...
        case BLK_EH_QUIESCED:
                set_bit(REQ_ATOM_QUIESCED, &req->atomic_flags);
                break;

CPU 1 takes an interrupt for the completion of the same request:

blk_complete_request():
        if (!blk_mark_rq_complete(req) ||  // this fails, as it's already marked complete
            test_and_clear_bit(REQ_ATOM_QUIESCED, &req->atomic_flags))  // this succeeds
                __blk_complete_request(req); // so we complete the request

Later, after the adapter reset is finished, you mentioned that all
requests will be completed.  My question is: will this result in a
double completion for this particular request?

I hope that's more clear.

-Jeff