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by any1 2363 days ago
According to "man vdso", futex is not in vDSO on any architecture. What eliminates the syscall cost is that it's rarely called due to elision.

A mutex may try to spin more than a "couple of times" before it calls futex. Example: https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_...
1 comments
rrss 2363 days ago
> is that it's rarely called due to elision.

Calling this 'elision' is a bit confusing, since the glibc developers use the term 'lock elision' to refer to hardware lock elision via Intel TSX extensions.

What eliminates the syscall cost is optimistic spinning in userspace, so that locking only does futex_wait in cases of contention when the lock isn't released 'soon' after the thread starts trying to acquire it.

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