Re: kern/52769: hang with an ffs stored in an nvme device

[mlelstv%serpens.de@localhost (Michael van Elst)]

The following reply was made to PR kern/52769; it has been noted by GNATS.

From: mlelstv%serpens.de@localhost (Michael van Elst)
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/52769: hang with an ffs stored in an nvme device
Date: Sat, 2 Dec 2017 09:51:03 -0000 (UTC)

 chuq%chuq.com@localhost (Chuck Silvers) writes:
 
 >the logic in dk_start() that allows two threads to be processing the bufq
 >in parallel looks like it could result in losing track of a buffer entirely.
 >if both threads get EAGAIN from d_diskstart then then will both try to
 >save the buffer pointer in dksc->sc_deferred, and the first one will be lost.
 
 Two threads cannot enter d_diskstart as the queue processing is skipped
 for sc_busy > 1. Instead the first thread will retry the queue (including
 a possible sc_deferred entry).
 
 -- 
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                                 Michael van Elst
 Internet: mlelstv%serpens.de@localhost
                                 "A potential Snark may lurk in every tree."