On 09/03/2014 01:56 PM, Greg Kroah-Hartman wrote: > On Wed, Sep 03, 2014 at 11:10:45AM -0400, Peter Hurley wrote: >> On 09/03/2014 08:19 AM, One Thousand Gnomes wrote: >>>> Ahh. Thanks for the insight, Alan. >>>> >>>> But set_bit() et. al. will generate an incredible amount of churn; >>>> what if I split the fields up to prevent false-sharing? >>> >>> Do you feel lucky ;-) >> >> Hahaha :) >> >>> I'd rather set_bit and friends were used. They exist largely for this >>> kind of reason and they also have atomic test/set methods which may in >>> the longer term be very useful. >>> >>> Yes it is churn can't argue with that. >> >> Yuck. There should be a better way. IXANY mode is suddenly going to >> have a ton of unnecessary bus locks on x86. > > True, but at least it will be correct, which I'm guessing today it isn't > :( > >> Note the ctrl_status field is a byte as well, which can't be RMW'ed by >> the bit-locked primitives, and definitely should not be aggregated with >> any adjacent field. > > Never trust what an ia64 compiler can, and will, do... I could not get the ia64 compiler to merge adjacent reads or writes on bools or chars, at all. However, Alan's point applies anyway -- because the Alpha is not byte-addressable. This means that smaller-than-int storage is _always_ read-modify-write. So my plan is now to take this patch out of this series and submit a new series on top of this one that will: 1) merge the ctrl_status field and packet field into an int size, which will guarantee atomicity to those fields on all arches when accessed holding the ->ctrl_lock. 2) make tty->hw_stopped an int. This way legacy drivers -- synclink, cyclades, nozomi, isdn4linux, et. al. -- will not need changes. UART drivers should use uport->hw_stopped (added in this series). New, non-UART drivers should use their own hw_stopped mechanism. 3) keep tty->stopped and tty->flow_stopped as bitfield members of the same bitfield, but size that bitfield to unsigned long [note 1 below]. This should guarantee atomicity on all arches when accessed while holding the ->flow_lock, without requiring further code changes and without impacting other arches. Regards, Peter Hurley [Note 1] gcc pre-4.7.2 corrupts data adjacent to a smaller-than-8-byte bitfield on sparc64, ia64, ppc64 and alpha. -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
