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Differential D8195

When downgrading exclusively busied page to shared-busy state, wakeup waiters
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Authored by kib on Oct 8 2016, 5:05 PM.
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rS307064: When downgrading exclusively busied page to shared-busy state, wakeup
Summary

... otherwise owners of the shared-busy state might get into a deadlock.

Test Plan

The vm_page_busy_downgrade() function is not used in the tree right now, but I have a plan for it.

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rS FreeBSD src repository - subversion
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kib updated this revision to Diff 21183.Oct 8 2016, 5:05 PM
kib retitled this revision from to When downgrading exclusively busied page to shared-busy state, wakeup waiters.
kib updated this object.
kib edited the test plan for this revision. (Show Details)
kib added reviewers: alc, markj.
kib set the repository for this revision to rS FreeBSD src repository - subversion.
Herald added a subscriber: imp. · View Herald TranscriptOct 8 2016, 5:05 PM
kib added a child revision: D8198: Buffer pager for FFS..Oct 8 2016, 5:29 PM
kib added a subscriber: pho.
markj accepted this revision.Oct 10 2016, 5:42 PM
markj edited edge metadata.
This revision is now accepted and ready to land.Oct 10 2016, 5:42 PM
alc added inline comments.Oct 11 2016, 4:45 PM
sys/vm/vm_page.c
669 ↗(On Diff #21183)

I'm confused here. It appears that we're waking up any waiters but leaving the waiters bit set in busy_lock. Do you really want to retain the second use of "| x" here if we're waking the waiters?

kib added inline comments.Oct 11 2016, 5:09 PM
sys/vm/vm_page.c
669 ↗(On Diff #21183)

Hm, we cannot wake up only shared waiters. Indeed, the new value for cmpset() can (and probably should) omit VPB_BIT_WAITERS. If exclusive-busy waiter exists, it must recheck the page state and re-set the VPB_BIT_WAITERS.

kib updated this revision to Diff 21260.Oct 11 2016, 5:14 PM
kib edited edge metadata.

Do not set VPB_BIT_WAITER for the new value, we are waking every waiting threads and it is their responsibility to detect stray wakeup.

This revision now requires review to proceed.Oct 11 2016, 5:14 PM
alc accepted this revision.Oct 11 2016, 5:32 PM
alc edited edge metadata.
This revision is now accepted and ready to land.Oct 11 2016, 5:32 PM
Closed by commit rS307064: When downgrading exclusively busied page to shared-busy state, wakeup (authored by kib). · Explain WhyOct 11 2016, 6:09 PM
This revision was automatically updated to reflect the committed changes.

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head/
sys/
vm/
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Diff 21263

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head/sys/vm/vm_page.c

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