Hello linux-block,
I wrote some tests recently to test patches against bsg.c and bsg-lib.c,
and while writing those I noticed something strange:
When you use the write() and read() call on multiple file-descriptors
for a single bsg-device (FC or SCSI), it is possible that you get
cross-talk between those different file-descriptors.
E.g.: You have two independent processes open a FD on /dev/bsg/fc_host0
and you send commands via write() in both processes, when they both do
read() later on - to read the response for the commands they send before
-, it is possible that process A gets the response (Sense-Data,
FC-Protocol-Data, ...) for a command that process B wrote and vis-a-vis.
I noticed this because in my tests I 'tag' each command I send with
write() via a value I write into the usr_ptr field of sg_io_v4. When I
later user read() to receive responses I check for this tag in a
hash-table and so can look up the original command. When I used this and
spawned multiple processes with the same target bsg-device, I got
responses for commands I don't have tags for in my hash-table, so they
were written by an other process. This never happend when I only spawn
one test-process.
This seems awfully contra-productive.. so much that I don't see any
point in allowing users to open more than one FD per bsg-device, because
that would inevitably lead to very hard to debug 'bugs'. And I wonder if
this is really by design or some regression that happend over time.
I looked at the code in bsg.c and yes, it seems this is what is coded
right now. We have a hash-table which manages all 'struct bsg_device's
who are indexed by device-minors, so one hash-table entry per
device-node.
So eh, long talk short question, is that intended?
Beste Grüße / Best regards,
- Benjamin Block
--
Linux on z Systems Development / IBM Systems & Technology Group
IBM Deutschland Research & Development GmbH
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