lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [day] [month] [year] [list]
Date:	Thu, 22 Nov 2007 19:07:24 -0600
From:	Robert Hancock <>
To:	Leon Woestenberg <>
Subject: Re: Use of mutex in interrupt context flawed/impossible, need advice.

Leon Woestenberg wrote:
> Hello,
> I'm converting an out-of-tree (*1) driver from binary semaphore to mutex.
> Userspace updates a look-up-table using write(). The driver tries to
> write this LUT to the FPGA in the (video frame) interrupt handler. It
> is important that the LUT is consistent and thus changed atomically.
> Note that it is not important that the LUT is updated each interrupt.
> The current approach is to try-down()ing a binary semaphore in
> interrupt context, and write the LUT to the FPGA if the semaphore was
> down()ed, do nothing else.
> The write() down()s the semaphore as well before updating the
> in-driver-copy of the LUT, then up()s it again.
> I understand this design is not clean (*2), and not even possible with
> mutexes, as mutex_trylock() is not interrupt safe.
> My current approach would be to have userspace write into a shadow
> copy, and use a spinlock to update the live copy. The interrupt then
> would try a spinlock.

Unless this update into the FPGA takes a significant amount of time, I 
wouldn't bother with that complexity - just do spin_lock_irq/irqsave on 
that spinlock.

Using a trylock for this rather sucks since the behavior is entirely 
non-deterministic. It could take a really long time in some cases for 
the trylock to ever succeed.

> My feeling is that we have a  valid use of mutex_trylock() in
> interrupt context; "i.e. update LUT if we can do so consistently and
> in time, or not at all".
> I would like to know why this is not so, and if someone has a cleaner
> proposal than the "try spinlock" approach?

Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from
Home Page:

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

Powered by blists - more mailing lists