[Bug 274535] vlc(1) from Focal hangs on futex

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274535

            Bug ID: 274535
           Summary: vlc(1) from Focal hangs on futex
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: trasz@FreeBSD.org

VLC from amd64 Ubuntu Focal "hangs" on futex on startup.  VLC from a previous
version (Bionic) doesn't exhibit this problem.

Running under strace(1) shows the following:

mmap(NULL, 16488, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = 0x804f88000
mmap(0x804f89000, 4096, PROT_READ|PROT_EXEC,
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0x1000) = 0x804f89000
mmap(0x804f8a000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4,
0x2000) = 0x804f8a000
mmap(0x804f8b000, 8192, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0x2000) = 0x804f8b000
close(4)                                = 0
mprotect(0x804f8b000, 4096, PROT_READ)  = 0
futex(0x80224ede8, FUTEX_WAKE_PRIVATE, 2147483647) = 0
futex(0x10b75b0, FUTEX_WAIT_BITSET_PRIVATE, 0, {tv_sec=0, tv_nsec=0},
FUTEX_BITSET_MATCH_ANYstrace: detach: tkill(2718,0): Invalid argument
strace: Process 2718 detached
 <detached ...>

Note that there are other applications hanging on futex, notably glxinfo(1). 
The strange thing is, the same glxinfo works when run under strace(1) or
FreeBSD's native ktrace(1).  Race condition of some kind?

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