waiteventtimeout

waiteventtimeout refers to the kernel macro wait_event_timeout, a synchronization primitive used in the Linux kernel. It blocks the calling task until a specified predicate evaluates to true or a timeout expires, whichever happens first. The macro works with a wait queue (wait_queue_head_t) and a condition expression, and it is commonly used to wait for an event such as I/O completion or state changes in driver code.

Mechanism and semantics: The macro suspends the current task and puts it on the provided wait queue. The task is woken up when either another thread or interrupt source signals the event by updating the condition or when the specified timeout elapses. After wake-up, the condition is re-evaluated, and the macro returns accordingly. The timeout is specified in kernel time units (jiffies), and there are helper conversions to specify time in milliseconds or seconds.

Return value: The macro returns a non-zero value if the predicate became true before the timeout. It returns 0 if the timeout elapsed without the condition becoming true. This simple boolean-style return makes it straightforward to handle both waiting and timeout cases in a single expression.

Usage notes: wait_event_timeout is intended for use in kernel code, within device drivers or subsystems that rely on wait queues. It is often paired with wake_up or wake_up_interruptible to awaken waiting tasks when the event occurs. The timeout parameter is typically provided as a jiffy count, with ms_to_jiffies or other conversions used to specify time in more human-friendly units.

See also: wait_queue_head_t, wait_event, wake_up, wake_up_interruptible, and other wait macros available in Linux kernel headers. These primitives collectively support efficient, race-free synchronization between producer and consumer components in kernel space.