poll: Write back only revents to userspace, matching Linux behavior#12851
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tanyifeng wants to merge 1 commit intogoogle:masterfrom
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poll: Write back only revents to userspace, matching Linux behavior#12851tanyifeng wants to merge 1 commit intogoogle:masterfrom
tanyifeng wants to merge 1 commit intogoogle:masterfrom
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Reproduction# In gVisor (runsc) — CPU spikes to ~95%
docker run -d --name tmux-test --runtime=runsc ubuntu:22.04 sleep 3600
docker exec tmux-test bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-test bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 94.7% CPU
# In Linux (runc) — CPU stays at 0%
docker run -d --name tmux-ctrl --runtime=runc ubuntu:22.04 sleep 3600
docker exec tmux-ctrl bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-ctrl bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 0.0% CPU |
EtiennePerot
reviewed
Apr 6, 2026
| // reventsOffsetInFD is the byte offset of the REvents field within | ||
| // linux.PollFD. Since REvents is the last field (int16), its offset | ||
| // equals the struct size minus the size of int16. | ||
| reventsOffsetInFD = sizeofPollFD - (*primitive.Int16)(nil).SizeBytes() |
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Can you use unsafe.Offsetof instead?
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Ok, I pushed the code to use unsafe.Offsetof(linux.PollFD{}.REvents), Thanks!
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: use an internal mask for POLLHUP|POLLERR in initReadiness() and pollBlock(); write back only the 2-byte revents field per fd in doPoll(). Signed-off-by: Tan Yifeng <yiftan@tencent.com>
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doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%).
Fix: use an internal mask for POLLHUP|POLLERR in initReadiness() and pollBlock(); write back only the 2-byte revents field per fd in doPoll().