Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

On Tue, Jan 20, 2015 at 02:40:32PM -0800, Andy Lutomirski wrote:

On Tue, Jan 20, 2015 at 1:57 AM, Fam Zheng [off-list ref] wrote:

quoted

This adds a new system call, epoll_mod_wait. It's described as below:

NAME
       epoll_mod_wait - modify and wait for I/O events on an epoll file
                        descriptor

SYNOPSIS

       int epoll_mod_wait(int epfd, int flags,
                          int ncmds, struct epoll_mod_cmd *cmds,
                          struct epoll_wait_spec *spec);

DESCRIPTION

       The epoll_mod_wait() system call can be seen as an enhanced combination
       of several epoll_ctl(2) calls, which are followed by an epoll_pwait(2)
       call. It is superior in two cases:

       1) When epoll_ctl(2) are followed by epoll_wait(2), using epoll_mod_wait
       will save context switches between user mode and kernel mode;

       2) When you need higher precision than microsecond for wait timeout.

       The epoll_ctl(2) operations are embedded into this call by with ncmds
       and cmds. The latter is an array of command structs:

           struct epoll_mod_cmd {

                  /* Reserved flags for future extension, must be 0 for now. */
                  int flags;

                  /* The same as epoll_ctl() op parameter. */
                  int op;

                  /* The same as epoll_ctl() fd parameter. */
                  int fd;

                  /* The same as the "events" field in struct epoll_event. */
                  uint32_t events;

                  /* The same as the "data" field in struct epoll_event. */
                  uint64_t data;

                  /* Output field, will be set to the return code once this
                   * command is executed by kernel */
                  int error;
           };

I would add an extra u32 at the end so that the structure size will be
a multiple of 8 bytes on all platforms.

*shrug*, but if you do so, enforce that it has a value of 0 or return
-EINVAL, just like a flags field.  Alternatively, move the last field
earlier and make flags a uint64_t.

- Josh Triplett