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epoll: edge-triggered descriptors lost their edges on more than one CPU

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EPOLLET reported a descriptor again only after a scan had seen it not
ready. Xorg watches its clients edge-triggered; when it read part of a
client's requests and more arrived before its next scan, the socket never
looked not-ready, so it was never reported again -- X waiting on the
client, the client waiting on X. One CPU almost never hit it; on a 16-thread
laptop the desktop froze within seconds: Window Maker's menu stuck, xterm
never mapped, text gone, and Caps Lock dead because X owns the LED.

Now a still-ready descriptor is reported again whenever anything has
signalled readiness since its last report (poll_generation()), which may be
spurious, as EPOLLET permits, and is never missed. Reproduced and verified
with QEMU at -smp 8: the root menu works and xterm runs cmatrix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
This commit is contained in:
Nexoniarz 2026-10-25 11:59:12 +01:00
commit 4743df8c4a
3 changed files with 25 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

18
kernel/sys/sys_event.c
View file

@ -82,6 +82,7 @@ struct epitem {
struct file *f; // checked against the table, never dereferenced alone
uint32 events;
uint64 data;
uint seq; // EPOLLET: poll_generation() when last reported
int was_ready; // for EPOLLET: report on becoming ready, not on being
int disabled; // EPOLLONESHOT fired, until EPOLL_CTL_MOD
};
@ -649,7 +650,8 @@ sys_epoll_ctl(void)
case EPOLL_CTL_ADD:
if (i >= 0) { r = -EEXIST; break; }
if (o->ep.n == EP_MAX) { r = -ENOSPC; break; }
o->ep.items[o->ep.n++] = (struct epitem){ fd, tf, ev.events, ev.data, 0, 0 };
o->ep.items[o->ep.n++] = (struct epitem){ .fd = fd, .f = tf, .events = ev.events,
.data = ev.data };
break;
case EPOLL_CTL_MOD:
if (i < 0) { r = -ENOENT; break; }
@ -676,6 +678,9 @@ static int
ep_ready(struct evobj *o, struct epoll_event *out, int max)
{
int got = 0;
// Taken before any descriptor is looked at, so a kick that lands during the
// scan is newer than what is recorded and is reported next time.
uint gen = poll_generation();
acquiresleep(&o->lock);
for (int i = 0; i < o->ep.n && got < max; ) {
@ -699,12 +704,21 @@ ep_ready(struct evobj *o, struct epoll_event *out, int max)
it->was_ready = 0;
continue;
}
if ((it->events & EPOLLET) && it->was_ready)
// Edge-triggered: an edge is new data, not only a not-ready-to-ready
// change. A descriptor still ready since it was last reported is
// reported again if anything has signalled since -- possibly spuriously,
// which EPOLLET allows; never missed, which it does not. Xorg watches its
// clients this way. Waiting for a not-ready scan in between lost the
// edge whenever X read part of a client's requests and more arrived
// before it looked again: on one CPU that almost never happened, on
// several it hung the desktop -- X waiting on the client, the client on X.
if ((it->events & EPOLLET) && it->was_ready && it->seq == gen)
continue;
if (out) {
out[got].events = (uint32)rev;
out[got].data = it->data;
it->was_ready = 1;
it->seq = gen;
if (it->events & EPOLLONESHOT)
it->disabled = 1;
}

8
kernel/sys/sys_poll.c
View file

@ -47,6 +47,14 @@ poll_kick(void)
wakeup(&ticks);
}
// How many kicks there have been. epoll's edge-triggered mode compares it:
// see ep_ready().
uint
poll_generation(void)
{
return __atomic_load_n(&poll_seq, __ATOMIC_ACQUIRE);
}
// Sleep one timer tick, or until a poll_kick, or until killed/signaled.
// Returns 1 if a tick went by, 0 if it was a kick -- which must not count
// against a timeout, or a timed poll on a busy network would end early --