On 03/04, Linus Torvalds wrote: > > On Tue, 4 Mar 2025 at 05:45, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > > Don't we need the trivial one-liner below anyway? > > See this email of mine: > > https://lore.kernel.org/all/CAHk-=wiCRwRFi0kGwd_Uv+Xv4HOB-ivHyUp9it6CNSmrKT4gOA@xxxxxxxxxxxxxx/ > > and the last paragraph in particular. > > The whole "poll_usage" thing is a kernel *hack* to deal with broken > user space that expected garbage semantics that aren't real, and were > never real. Yes agreed. But we can make this hack more understandable. But as I said, this is off-topic right now. > introduced that completely bogus hack to say "ok, we'll send these > completely wrong extraneous events despite the fact that nothing has > changed, because some broken user space program was written to expect > them". Yes, but since we already have this hack: > That program is buggy, and we're not adding new hacks for new bugs. Yes, but see below... > If you ask for an edge-triggered EPOLL event, you get an *edge* > triggered EPOLL event. And there is no edge - the pipe was already > readable, no edge anywhere in sight. Yes, the pipe was already readable before before fork, but this condition was already "consumed" by the 1st epoll_wait(EPOLLET). Please see below. > If anything, we might consider removing the crazy "poll_usage" hack in > the (probably futile) hope that user space has been fixed. This would be the best option ;) Until then: I agree that my test case is "buggy", but afaics it is not buggier than userspace programs which rely on the unconditional kill_fasync()'s in pipe_read/pipe_write? So. anon_pipe_write() does if (was_empty) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); before wait_event(pipe->wr_wait), but before return it does if (was_empty || pipe->poll_usage) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); and this looks confusing to me. If pipe_write() doesn't take poll_usage into account before wait_event(wr_wait), then it doesn't need kill_fasync() too? So I won't argue, but why not make both cases more consistent? Oleg.
