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From: Brian Geffon <>
To: Andrea Arcangeli <>
Cc: linux-mm <>,
	LKML <>,
	 Sonny Rao <>, Peter Xu <>
Subject: Userfaultfd doesn't seem to break out of poll on fd close
Date: Sun, 12 Apr 2020 13:10:40 -0700	[thread overview]
Message-ID: <> (raw)

It seems that userfaultfd isn't woken from a poll when the file
descriptor is closed. It seems that it should be from the code in
userfault_ctx_release, but it appears that's not actually called
immediately. I have a simple standalone example that shows this
behavior. It's straight forward: one thread creates a userfaultfd and
then closes it after a second thread has entered a poll syscall, some
abbreviated strace output is below showing this and the code can be
seen here:

Given that it's probably very common to have a dedicated thread remain
blocked indefinitely in a poll(2) waiting for faults there must be a
way to break it out early when it's closed. Am I missing something?


// Open a userfaultfd from Thread 1.
12:55:27.611942 userfaultfd(O_NONBLOCK) = 3
12:55:27.612007 ioctl(3, UFFDIO_API, {api=0xaa, features=0 =>

// Create a second thread (Thread 2) to poll.
12:55:27.612447 clone(strace: Process 72730 attached
parent_tidptr=0x7f30efaa09d0, tls=0x7f30efaa0700,
child_tidptr=0x7f30efaa09d0) = 72730

// Thread 2 will poll on the userfaultfd for up to 2000ms.
[pid 72730] 12:55:27.612676 poll([{fd=3, events=POLLIN}], 1, 2000
<unfinished ...>

// Thread 1 closes the userfaultfd and fcntl confirms it's closed:
[pid 72729] 12:55:28.612945 close(3)    = 0
[pid 72729] 12:55:28.613039 fcntl(3, F_GETFD) = -1 EBADF (Bad file descriptor)

// Poll technically times out and while loop back in do_poll it gets a POLLNVAL.
[pid 72730] 12:55:29.614906 <... poll resumed> ) = 1 ([{fd=3,

             reply	other threads:[~2020-04-12 20:11 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-12 20:10 Brian Geffon [this message]
2020-04-14 21:45 ` Userfaultfd doesn't seem to break out of poll on fd close Peter Xu
2020-04-14 22:34   ` Jason Gunthorpe
2020-04-15  3:16   ` Hillf Danton
2020-04-15 14:25     ` Jason Gunthorpe
2020-04-15 15:16       ` Brian Geffon
2020-04-16  0:02       ` Andrea Arcangeli
2020-04-16  1:15         ` Brian Geffon
2020-04-16  1:37           ` Peter Xu
2020-04-16  4:39             ` Brian Geffon
2020-04-16 14:49           ` Jason Gunthorpe

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