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block: make BlockBackend->quiesce_counter atomic
The main loop thread increments/decrements BlockBackend->quiesce_counter when drained sections begin/end. The counter is read in the I/O code path. Therefore this field is used to communicate between threads without a lock. Acquire/release are not necessary because the BlockBackend->in_flight counter already uses sequentially consistent accesses and running I/O requests hold that counter when blk_wait_while_drained() is called. qatomic_read() can be used. Use qatomic_fetch_inc()/qatomic_fetch_dec() for modifications even though sequentially consistent atomic accesses are not strictly required here. They are, however, nicer to read than multiple calls to qatomic_read() and qatomic_set(). Since beginning and ending drain is not a hot path the extra cost doesn't matter. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Message-Id: <20230307210427.269214-2-stefanha@redhat.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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