Re: [Qemu-devel] [PATCH 4/8] block: add a Rwlock to synchronous read/write drivers

[Kevin Wolf <kwolf@redhat.com>]

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 <pbonzini@redhat.com>

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