The bug is caused by this patch:
659743b02c411075b26601725947b21df0bb29c8
which allowed the task lists to be manipulated under different locks
in the xmit and completion path.
To fix the oops this patch just reverts that patch. It also reverts
these 2 patches for regressions that were also a result of that patch:
Whoa now Mike, this is a major change. Can we take a step back and think
about this for a second?
My understanding is that the kfifo circular buffer design allows a
writer (e.g. the producer) and a reader (e.g. the consumer) to avoid
extra locking when accessing the kfifo buffer.
From include/linux/kfifo.h:
/*
* Note about locking : There is no locking required until only * one
reader
* and one writer is using the fifo and no kfifo_reset() will be * called
* kfifo_reset_out() can be safely used, until it will be only called
* in the reader thread.
* For multiple writer and one reader there is only a need to lock the
writer.
* And vice versa for only one writer and multiple reader there is only
a need
* to lock the reader.
*/
The patch by Shlomo, implements the locking policy documented above:
- multiple readers: multiple threads entering queuecommand reading the
kfifo (kfifo_out) are mutually excluded by the frwd_lock.
- multiple writers: completion contexts writing to the kfifo (kfifo_in)
are mutually excluded by the back_lock.
Can you provide a more detailed analysis of why is this list corruption
triggered? What scenario was not honoring the locking policy?
This code has been running reliably in our labs for a long time now
(both iser and tcp).
Going back to a single lock restores the contention point between
queuecommand and complete_pdu.
P.S.
While we're on the subject, I'd like to see block tags replace the
kfifo for iscsi tasks. It's difficult because sw and offload drivers
map hosts/sessions differently. It can be done if we move the tasks to
the iscsi_host and have a reserved (unique) task tags for the session
login/logout/nop_in/nop_out. With this we won't need locks around our
tasks arrays.
Sagi.
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