Hi, [please CC me, I'm not subscribed to the list]
I'm writing a driver for a pretty simple USB device, and most of what I need I can see in similar drivers; right now I'm lifting a lot of code from drivers/usb/class/usblp.c. In the read routine, this driver gathers a few locks, then checks whether data is present and in the non-blocking case returns -EAGAIN, otherwise waits for data to become available. In the case of multiple readers, acquiring the locks might however block, so I'm not sure whether it'd be better to check for non-blocking first, and acquire the locks with trylock in this case, returning -EAGAIN in case we fail to acquire any. The semantic difference seems to be minimal: I believe that it is acceptable to return -EAGAIN from read even if a previous poll() suggested that the fd was now readable. Blocking on a non-blocking read() until other tasks reading from the same fd have left the kernel seems more like an optimization to me, saving a few syscalls if the other reader doesn't read all the bytes. The obvious consistency problem with multiple unsynchronized readers aside: is that optimization worth a (short) block inside a non-blocking read? Simon -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
