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[Qemu-devel] Re: [PATCH 2/2] vnc: don't mess up with iohandlers in the v

From: Paolo Bonzini
Subject: [Qemu-devel] Re: [PATCH 2/2] vnc: don't mess up with iohandlers in the vnc thread
Date: Thu, 10 Mar 2011 14:56:05 +0100
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On 03/10/2011 02:45 PM, Anthony Liguori wrote:
On 03/10/2011 07:06 AM, Paolo Bonzini wrote:
On 03/10/2011 01:59 PM, Corentin Chary wrote:
Instead, we now store the data in a temporary buffer, and use a socket
pair to notify the main thread that new data is available.

You can use a bottom half for this instead of a special socket.
Signaling a bottom half is async-signal- and thread-safe.

Bottom halves are thread safe?

I don't think so.

They probably should be but they aren't today.

Creating a new bottom half is not thread-safe, but scheduling one is. Assuming that you never use qemu_bh_schedule_idle, qemu_bh_schedule boils down to:

    if (bh->scheduled)
    bh->scheduled = 1;
    /* stop the currently executing CPU to execute the BH ASAP */

You may have a spurious wakeup if two threads race on the same bottom half (including the signaling thread racing with the IO thread), but overall you can safely treat them as thread-safe.


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