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Re: [Qemu-devel] [PATCH 4/8] block: add a Rwlock to synchronous read/wri
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 4/8] block: add a Rwlock to synchronous read/write drivers |
Date: |
Thu, 20 Oct 2011 11:47:48 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
Am 19.10.2011 16:59, schrieb Paolo Bonzini:
> The big conversion of bdrv_read/write to coroutines caused the two
> homonymous callbacks in BlockDriver to become reentrant. It goes
> like this:
>
> 1) bdrv_read is now called in a coroutine, and calls bdrv_read or
> bdrv_pread.
>
> 2) the nested bdrv_read goes through the fast path in bdrv_rw_co_entry;
>
> 3) in the common case when the protocol is file, bdrv_co_do_readv calls
> bdrv_co_readv_em (and from here goes to bdrv_co_io_em), which yields
> until the AIO operation is complete;
>
> 4) if bdrv_read had been called from a bottom half, the main loop
> is free to iterate again: a device model or another bottom half
> can then come and call bdrv_read again.
>
> This applies to all four of read/write/flush/discard. It would also
> apply to is_allocated, but it is not used from within coroutines:
> besides qemu-img.c and qemu-io.c, which operate synchronously, the
> only user is the monitor. Copy-on-read will introduce a use in the
> block layer, and will require converting it.
>
> The solution is "simply" to convert all drivers to coroutines! We
> have nothing to do for the read-only drivers. For the others, we
> add a Rwlock that is taken around affected operations.
>
> Signed-off-by: Paolo Bonzini <address@hidden>
For the record, we just discussed this on IRC: We'll probably need a
mutex in some/most cases instead. Also, cloop and dmg need locking. That
leaves only bochs and parallels that should be okay without a lock.
We decided to play it the safe way and add locking in every driver and
make it a mutex everywhere.
Kevin
- [Qemu-devel] [PATCH 0/8] finish coroutinization of drivers, Paolo Bonzini, 2011/10/19
- [Qemu-devel] [PATCH 4/8] block: add a Rwlock to synchronous read/write drivers, Paolo Bonzini, 2011/10/19
- Re: [Qemu-devel] [PATCH 4/8] block: add a Rwlock to synchronous read/write drivers,
Kevin Wolf <=
- [Qemu-devel] [PATCH 5/8] block: take lock around bdrv_read implementations, Paolo Bonzini, 2011/10/19
- [Qemu-devel] [PATCH 2/8] vmdk: fix return values of vmdk_parent_open, Paolo Bonzini, 2011/10/19
- [Qemu-devel] [PATCH 6/8] block: take lock around bdrv_write implementations, Paolo Bonzini, 2011/10/19
- [Qemu-devel] [PATCH 7/8] block: change flush to co_flush, Paolo Bonzini, 2011/10/19
- [Qemu-devel] [PATCH 8/8] block: change discard to co_discard, Paolo Bonzini, 2011/10/19