On Mon, May 02, 2022 at 09:54:14AM +0200, Emanuele Giuseppe Esposito wrote: > > > Am 30/04/2022 um 07:48 schrieb Stefan Hajnoczi: > > On Fri, Apr 29, 2022 at 10:37:54AM +0200, Emanuele Giuseppe Esposito wrote: > >> Am 28/04/2022 um 15:45 schrieb Stefan Hajnoczi: > >>> On Tue, Apr 26, 2022 at 04:51:09AM -0400, Emanuele Giuseppe Esposito > >>> wrote: > >>>> +static int has_writer; > >>> > >>> bool? > >> > >> Yes and no. With the latest findings and current implementation we could > >> have something like: > >> > >> wrlock() > >> has_writer = 1 > >> AIO_WAIT_WHILE(reader_count >=1) --> job_exit() > >> wrlock() > >> > >> But we are planning to get rid of AIO_WAIT_WHILE and allow wrlock to > >> only run in coroutines. This requires a lot of changes, and switch a lot > >> of callbacks in coroutines, but then we would avoid having such problems > >> and nested event loops. > > > > I don't understand how this answer is related to the question about > > whether the type of has_writer should be bool? > > Yes sorry I did not conclude the explanation, but taking into account > the above case we would have an assertion failure `assert(!has_writer)` > in bdrv_graph_wrlock(), and just removing that would make the lock > inconsistent because the first unlock() would reset the flag to > zero/false and forget about the previous wrlock(). > Example: > > wrlock() > has_writer = 1 > AIO_WAIT_WHILE(reader_count >=1) --> job_exit() > wrlock() > has_writer = 1 > /* performs a write */ > wrunlock() > has_writer = 0 > <--- > /* performs a write but has_writer = 0! */
How is this related to the question of whether has_writer should be bool instead of int? Are you saying has_writer needs to be a recursive lock and is therefore a counter? If yes, please revisit the cover letter, which says the lock must not be recursive. I'm confused. Stefan
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