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| From: | Linus Torvalds |
| Subject: | Re: [Qemu-devel] [PATCH 5/7] userfaultfd: switch to exclusive wakeup for blocking reads |
| Date: | Mon, 15 Jun 2015 08:19:07 -1000 |
On Jun 15, 2015 7:22 AM, "Andrea Arcangeli" <address@hidden> wrote:
>
> Blocking reads can easily use exclusive wakeups. Poll in theory could
> too but there's no poll_wait_exclusive in common code yet.
NAK.
Tie while commit message is crap, and so us the comment
No, your really cannot "easily use exclusive waits", and no, using them for polling isn't about a lack of interface, it's about the fact that it would be buggy shit.
What if the process doing the polling never doors anything with the end result? Maybe it meant to, but it got killed before it could? Are you going to leave everybody else blocked, even though there are pending events?
The same us try of read() too. What if the reader only reads party of the message? The wake didn't wake anybody else, so now people are (again) blocked despite there being data.
So no, exclusive waiting is never "simple". You have to 100% guarantee that you will consume all the data that caused the wake event (or perhaps wake the next person up if you don't).
Linus
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