Date: Wed, 17 Oct 2018 23:54:18 +0200 From: Florian Weimer <fw@...eb.enyo.de> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: Possible design for global thread list * Rich Felker: > On Sun, Oct 14, 2018 at 07:32:54PM +0200, Florian Weimer wrote: >> * Rich Felker: >> >> >> > Of course, this futex wake is already used for pthread_join, which >> > would need another mechanism. This is solved simply: pthread_exit can >> > FUTEX_REQUEUE a waiting joiner to the thread-list lock. pthread_join >> > then has to wait on (but need not acquire) the thread-list lock after >> > waiting on the thread's own exit futex in order to ensure the exit has >> > actually finished. This is potentially subject to long waits if the >> > lock is under contention (lots of threads exiting or being created) >> > and retaken before pthread_join gets to run, but the probability of >> > collision can be made negligible (only possible under extremely rapid >> > tid reuse) by using the tid of the exiting thread as the wait value. >> > Alternatively, the tid of the joiner could be used, making collisions >> > impossible, but setting up to do this is more complex. >> >> I'm not sure if this is compatible with existing software which >> rapidly joins and creates many threads in succession because it looks >> to me that the pthread_join operation can return before the kernel >> resources are freed. As a result, applications will get impossible >> EAGAIN failures, even though the application never exceeds the thread >> limit. >> >> Depending on kernel version and cgroups configuration, this race can >> even be observed with the more usual join sequence because the kernel >> signals thread exit too early to user space. > > I think you must be confused about something, because either way, what > pthread_join is waiting for is the same kind of futex wake by the > kernel, and happens at the same point during kernel task exit. The > only difference is what address it's at. Right, I was confused.
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