Concurrent mount/unmount results in hanging about go-fuse HOT 15 CLOSED

thanks for your report. This is an interesting trace. It shows that it's calling fusermount despite specifying DirectMount. This is because there is a fallback; did you try using StrictDirectMount ?

Subprocessing with FUSE is a known source of deadlocks, see https://pkg.go.dev/github.com/hanwen/go-fuse/v2/fs#hdr-Deadlocks. Obviously, the umount operation should work, but if you force direct mounting, I think the problem will go away.

I'll try to have a detailed look later this week.

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Switching to DirectMountStrict: true indeed fixes things. Thanks!

from go-fuse.

Some debugging:

• problem is related to the subprocess for the mount; the unmount seems OK
• problem goes away if we disable the poll hack

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found a trace here:

syscall.Syscall6(0x100c000143818?, 0x7f7bbc608718?, 0xc0000cc4e0?, 0xc000034000?, 0x47e970?, 0xc0000b5bb0?, 0xc0000b5c28?)
	/usr/lib/golang/src/syscall/syscall_linux.go:91 +0x30 fp=0xc0000b5b70 sp=0xc0000b5ae8 pc=0x47e970
syscall.Syscall6(0x10f, 0xc0000b5ce0, 0x1, 0xc0000b5c90, 0x0, 0x0, 0x0)
	<autogenerated>:1 +0x3d fp=0xc0000b5bb8 sp=0xc0000b5b70 pc=0x47eefd
golang.org/x/sys/unix.ppoll(0xc0000225c0?, 0x38?, 0x38?, 0xc0000225c0?)
	/home/hanwen/go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_linux.go:124 +0x57 fp=0xc0000b5c38 sp=0xc0000b5bb8 pc=0x5391f7
golang.org/x/sys/unix.Ppoll({0xc0000b5ce0?, 0x0?, 0xc0000225c0?}, 0x0?, 0xffffffffffffff9c?)
	/home/hanwen/go/pkg/mod/golang.org/x/[email protected]/unix/syscall_linux.go:148 +0x38 fp=0xc0000b5c68 sp=0xc0000b5c38 pc=0x539078
golang.org/x/sys/unix.Poll({0xc0000b5ce0?, 0xc0000225c0?, 0xc000067438?}, 0x10?)
	/home/hanwen/go/pkg/mod/golang.org/x/[email protected]/unix/syscall_linux.go:157 +0x8d fp=0xc0000b5cb0 sp=0xc0000b5c68 pc=0x53914d
github.com/hanwen/go-fuse/v2/fuse.pollHack({0xc0000203c0?, 0xc00012aac0?})
	/home/hanwen/vc/go-fuse/fuse/poll_linux.go:22 +0xaf fp=0xc0000b5d18 sp=0xc0000b5cb0 pc=0x543ecf
github.com/hanwen/go-fuse/v2/fuse.(*Server).WaitMount(0xc000161ba0)

//sys   ppoll(fds *PollFd, nfds int, timeout *Timespec, sigmask *Sigset_t) (n int, err error)

func Ppoll(fds []PollFd, timeout *Timespec, sigmask *Sigset_t) (n int, err error) {
        if len(fds) == 0 {
                return ppoll(nil, 0, timeout, sigmask)
        }
        return ppoll(&fds[0], len(fds), timeout, sigmask)
}

if ppoll has special support for nonblocking i/o again, we are hosed again.

from go-fuse.

This is probably the scenario that involves dup3 documented at https://pkg.go.dev/github.com/hanwen/go-fuse/v2/fs#hdr-Deadlocks

When writing that section, I didn´t realize that fusermount itself uses inherited file descriptors.

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further detail of why the pollhack is involved: consider 2 threads (t1, t2). If t1 initiates the pollhack, it might get fd 3 for the poll hack. Then, if t2 calls fusermount, the mechanism above causes a FLUSH on the pollhack fd.

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see https://review.gerrithub.io/c/hanwen/go-fuse/+/1186976

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Posting on Gerrithub failed for some obscure reason [1] so posting here:

I tested https://review.gerrithub.io/c/hanwen/go-fuse/+/1186976 . TestParallelMount passes with the init() change and hangs without it. LGTM.

[1]:

An error occurred
Error submitting review Error: selector 'gr-label-scores' did not match anything'

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PS: gocryptfs already has something similar for different reasons. It's so things don't completely explode when fds 0,1,2 are not open.

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thanks, I wonder what is up with gerrithub. I couldnt add you to the review yesterday.

the "collapse" button on the reply dialog is also broken.

@lucamilanesio

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fixed in 36b3591

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i've tagged v2.5.1 for this.

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The open question is: why doesn't DirectMount work? I looked at the code and I don't see how this can do the wrong thing.

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not an open question. I think the tests were run as a normal user, so DirectMount fails with permission error. Then we fallback to fusermount.

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