linux-kernel - BUG in scsi_lib.c due to a bad commit

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Date:	Wed, 12 Nov 2014 00:33:02 +0100
From:	Barto <mister.freeman@...oste.net>
To:	linux-kernel@...r.kernel.org
Subject: BUG in scsi_lib.c due to a bad commit

Hello everyone,

I notice a bug since kernel 3.17 ( and also with 3.18 branch ), a random
hang at boot on some PC configurations, I did a "git bisect" and I found
that the culprit is  :

[045065d8a300a37218c548e9aa7becd581c6a0e8] [SCSI] fix qemu boot hang problem

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=045065d8a300a37218c548e9aa7becd581c6a0e8

the author of this commit has choosen to inverse the logic of the if
statement in the file  drivers/scsi/scsi_lib.c in order to solve an
issue with qemu :

--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1774,7 +1774,7 @@ static void scsi_request_fn(struct request_queue *q)
blk_requeue_request(q, req);
atomic_dec(&sdev->device_busy);
out_delay:
- if (atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
+ if (!atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
blk_delay_queue(q, SCSI_QUEUE_DELAY);
}

this change triggers a bug on my PC ( I don't have SCSI devices, but
only 3 SATA harddisks and 2 IDE harddisks, SATA disks are on an ICH7
sata controler on the motherboard gigabyte GA-P31-DS3L, and IDE disk on
a JMicron JMB363/368 Sata/IDE PCIe card ), every 5~10 boots the boot
stops suddenly because of this commit,

If I revert this commit then the bug is gone, more details can be found
here, where I created a patch who reverts commit 045065d8 :

https://bugzilla.kernel.org/show_bug.cgi?id=87581

my question: why Guenter Roeck ( the author of the bad commit ) has
choosen to inverse the logic in the if statement ?
before his commit the if statement was like this :

if (atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
blk_delay_queue(q, SCSI_QUEUE_DELAY);

if his decision to inverse the logic of 
"atomic_read(&sdev->device_busy)" is acceptable then the real bug is
probably located elsewhere in the scsi source code, and we must solve
this mistery because there is obviously a bug regression in SCSI code
because with older kernels ( 3.16.7 and lower ) I don't have the random
hang boot bug with my configuration,

another user in archlinux forums has also this bug and he has a more
modern PC ( intel i7 core cpu, SSD device ), my fear is when linux
distros will move to kernel 3.17 then more users will have this weird
random bug who can occur only on boot and only with a specific PC
configuration, if the boot step is passed despite the random bug then
the bug will not occur, it occurs only during the boot process, which
probably means that the faulty source code is only called during the
boot process,

thanks for anyone who wants to dig this problem with me
--
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