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qemu-thread: optimize QemuLockCnt with futexes on Linux

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This is complex, but I think it is reasonably documented in the source.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170112180800.21085-5-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2017-01-12 19:07:54 +01:00 • committed by Stefan Hajnoczi
parent d7c99a1282
commit fbcc3e5004
7 changed files with 342 additions and 35 deletions
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9
docs/lockcnt.txt
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@ -142,12 +142,11 @@ can also be more efficient in two ways:
- it avoids taking the lock for many operations (for example
incrementing the counter while it is non-zero);
- on some platforms, one could implement QemuLockCnt to hold the
lock and the mutex in a single word, making it no more expensive
- on some platforms, one can implement QemuLockCnt to hold the lock
and the mutex in a single word, making the fast path no more expensive
than simply managing a counter using atomic operations (see
docs/atomics.txt). This is not implemented yet, but can be
very helpful if concurrent access to the data structure is
expected to be rare.
docs/atomics.txt). This can be very helpful if concurrent access to
the data structure is expected to be rare.
Using the same mutex for frees and writes can still incur some small