On Sat, 24 Mar 2018 04:43:42 -0400, Jeffrey Walton said:
> My question is, is a futex with a 2147483647 or -1 expected? Where
> should I be looking to begin to narrow the problem down?
'man 7 futex' says:
Semantics
Any futex operation starts in user space, but it may be necessary to
communicate with the kernel using the futex(2) system call.
To "up" a futex, execute the proper assembler instructions that will
cause the host CPU to atomically increment the integer. Afterward,
check if it has in fact changed from 0 to 1, in which case there were
no waiters and the operation is done. This is the noncontended case
which is fast and should be common.
In the contended case, the atomic increment changed the counter from -1
(or some other negative number). If this is detected, there are wait-
ers. User space should now set the counter to 1 and instruct the ker-
nel to wake up any waiters using the FUTEX_WAKE operation.
Waiting on a futex, to "down" it, is the reverse operation. Atomically
decrement the counter and check if it changed to 0, in which case the
operation is done and the futex was uncontended. In all other circum-
stances, the process should set the counter to -1 and request that the
kernel wait for another process to up the futex. This is done using
the FUTEX_WAIT operation.
The futex(2) system call can optionally be passed a timeout specifying
how long the kernel should wait for the futex to be upped. In this
case, semantics are more complex and the programmer is referred to
futex(2) for more details. The same holds for asynchronous futex waiting.
which pretty much says that a negative value means there's somebody waiting
on the futex.
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