-----Original Message-----
From: Chip Bilbrey [mailto:chip at bilbrey.org]
Sent: Monday, November 6, 2017 12:33 AM
To: Oleksandr Shamray <redacted>
Cc: gregkh at linuxfoundation.org; arnd at arndb.de; linux-
kernel at vger.kernel.org; linux-arm-kernel at lists.infradead.org;
devicetree at vger.kernel.org; openbmc at lists.ozlabs.org; joel at jms.id.au;
jiri at resnulli.us; tklauser at distanz.ch; linux-serial at vger.kernel.org;
mec at shout.net; Vadim Pasternak [off-list ref]; system-sw-low-
level [off-list ref]; robh+dt at kernel.org; openocd-
devel-owner at lists.sourceforge.net; linux-api at vger.kernel.org;
davem at davemloft.net; mchehab at kernel.org; Jiri Pirko [off-list ref]
Subject: Re: [v11,1/4] drivers: jtag: Add JTAG core driver


Oleksandr Shamray writes:

[..]

I notice the single-open()-per-device lock was dropped by request in an earlier
revision of your patches, but multiple processes trying to drive a single JTAG
master could wreak serious havoc if transactions get interleaved. Would
something like an added JTAG_LOCKCHAIN/UNLOCKCHAIN
ioctl() for exclusive client access be reasonable to prevent this?

Yes, it dropped by recommendation of Greg KH [off-list ref]. 

Greg, what you can suggest about it. May be better to add again single-open()-per-device lock with right locking way like:

if (mutex_lock_interruptible(&jtag->open_lock)) {
return -ERESTARTSYS;
}

if (jtag->opened) {
mutex_unlock(&jtag->open_lock);
return -EINVAL;
}

nonseekable_open(inode, file);
file->private_data = jtag;
jtag->opened++;
mutex_unlock(&jtag->open_lock);

Thaks.

-Chip