17.8.3. poll
The poll system call allows an user process to do a non-busy wait on a kernel event.
Sources:
Example:
./poll.sh
Outcome: jiffies gets printed to stdout every second from userland, e.g.:
poll <6>[ 4.275305] poll <6>[ 4.275580] return POLLIN revents = 1 POLLIN n=10 buf=4294893337 poll <6>[ 4.276627] poll <6>[ 4.276911] return 0 <6>[ 5.271193] wake_up <6>[ 5.272326] poll <6>[ 5.273207] return POLLIN revents = 1 POLLIN n=10 buf=4294893588 poll <6>[ 5.276367] poll <6>[ 5.276618] return 0 <6>[ 6.275178] wake_up <6>[ 6.276370] poll <6>[ 6.277269] return POLLIN revents = 1 POLLIN n=10 buf=4294893839
Force the poll file_operation to return 0 to see what happens more clearly:
./poll.sh pol0=1
Sample output:
poll <6>[ 85.674801] poll <6>[ 85.675788] return 0 <6>[ 86.675182] wake_up <6>[ 86.676431] poll <6>[ 86.677373] return 0 <6>[ 87.679198] wake_up <6>[ 87.680515] poll <6>[ 87.681564] return 0 <6>[ 88.683198] wake_up
From this we see that control is not returned to userland: the kernel just keeps calling the poll file_operation again and again.
Typically, we are waiting for some hardware to make some piece of data available available to the kernel.
The hardware notifies the kernel that the data is ready with an interrupt.
To simplify this example, we just fake the hardware interrupts with a kthread that sleeps for a second in an infinite loop.
Bibliography: