Date: Fri, 15 Feb 2019 01:54:09 +0300 From: Alexey Izbyshev <izbyshev@...ras.ru> To: Rich Felker <dalias@...ifal.cx> Cc: musl@...ts.openwall.com Subject: Re: Draft outline of thread-list design On 2019-02-15 01:32, Rich Felker wrote: > On Fri, Feb 15, 2019 at 12:16:39AM +0300, Alexey Izbyshev wrote: >> On 2019-02-12 21:26, Rich Felker wrote: >> >pthread_join: >> > >> >A joiner can no longer see the exit of the individual kernel thread >> >via the exit futex (detach_state), so after seeing it in an exiting >> >state, it must instead use the thread list to confirm completion of >> >exit. The obvious way to do this is by taking a lock on the list and >> >immediately releasing it, but the actual taking of the lock can be >> >elided by simply doing a futex wait on the lock owner being equal to >> >the tid (or an exit sequence number if we prefer that) of the exiting >> >thread. In the case of tid reuse collisions, at worse this reverts to >> >the cost of waiting for the lock to be released. >> > >> Since the kernel wakes only a single thread waiting on ctid address, >> wouldn't the joiner still need to do a futex wake to unblock other >> potential waiters even if it doesn't actually take the lock by >> changing *ctid? > > I'm not sure. If it's just a single wake rather than a broadcast then > yes, but only if it waited. If it observed the lock word != to the > exiting thread tid without performing a futex wait then it doesn't > have to do a futex wake. > Yes, it's a single wake: <http://man7.org/linux/man-pages/man2/set_tid_address.2.html>, <https://elixir.bootlin.com/linux/v4.20.8/source/kernel/fork.c#L1292>. > >> In general, to my limited expertise, the design looks simple and >> clean. I'm not sure whether it's worth optimizing to reduce >> serialization pressure on pthread_create()/pthread_exit() because >> creating a large amount of short-lived threads doesn't look like a >> good idea anyway. > > Yes. One thing I did notice is that the window where pthread_create > has to hold a lock to prevent new dlopen from happening is a lot > larger than the window where the thread list needs to be locked, and > contains mmap/mprotect. I think we should add a new "DTLS lock" here > that's held for the whole time, with a protocol that if you need both > the DTLS lock and the thread list lock, you take them in that order > (dlopen would also need them both). This reduces the thread list lock > window to just the __clone call and list update. > Looks good. Alexey
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