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There exists a race condition when using wait_queue_head_t objects
that are declared on the stack. This was being done in a few places
where we are sending work requests to the FW and awaiting replies, but
we don't have an endpoint structure with an embedded c4iw_wr_wait
struct. So the code was allocating it locally on the stack. Bad
design. The race is:
1) thread on cpuX declares the wait_queue_head_t on the stack, then
posts a firmware WR with that wait object ptr as the cookie to be
returned in the WR reply. This thread will proceed to block in
wait_event_timeout() but before it does:
2) An interrupt runs on cpuY with the WR reply. fw6_msg() handles
this and calls c4iw_wake_up(). c4iw_wake_up() sets the condition
variable in the c4iw_wr_wait object to TRUE and will call
wake_up(), but before it calls wake_up():
3) The thread on cpuX calls c4iw_wait_for_reply(), which calls
wait_event_timeout(). The wait_event_timeout() macro checks the
condition variable and returns immediately since it is TRUE. So
this thread never blocks/sleeps. The function then returns
effectively deallocating the c4iw_wr_wait object that was on the
stack.
4) So at this point cpuY has a pointer to the c4iw_wr_wait object
that is no longer valid. Further its pointing to a stack frame
that might now be in use by some other context/thread. So cpuY
continues execution and calls wake_up() on a ptr to a wait object
that as been effectively deallocated.
This race, when it hits, can cause a crash in wake_up(), which I've
seen under heavy stress. It can also corrupt the referenced stack
which can cause any number of failures.
The fix:
Use struct completion, which supports on-stack declarations.
Completions use a spinlock around setting the condition to true and
the wake up so that steps 2 and 4 above are atomic and step 3 can
never happen in-between.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
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