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Rob Hoes
e8464b9533
Ocaml has a heap lock which must be held whenever ocaml code is running. Ocaml usually drops this lock when it enters a potentially blocking low-level function, such as writing to a file. Libxl has its own lock, which it may acquire when being called. Things get interesting when libxl calls back into ocaml code. There is a risk of ending up in a deadlock when a thread holds both locks at the same time, then temporarily drop the ocaml lock, while another thread calls another libxl function. To avoid deadlocks, we drop the ocaml heap lock before entering libxl, and reacquire it in callbacks to ocaml. This way, the ocaml heap lock is never held together with the libxl lock, except in osevent registration callbacks, and xentoollog callbacks. If we guarantee to not call any libxl functions inside those callbacks, we can avoid deadlocks. This patch handle the dropping and reacquiring of the ocaml heap lock by the caml_enter_blocking_section and caml_leave_blocking_section functions, and related macros. We are also careful to not call any functions that access the ocaml heap while the ocaml heap lock is dropped. This often involves copying ocaml values to C before dropping the ocaml lock. The ao_how in aohow_val is now malloc'ed, just to make this function a little easier to use. Signed-off-by: Rob Hoes <rob.hoes@citrix.com> Acked-by: David Scott <dave.scott@eu.citrix.com> Acked-by: Ian Campbell <ian.campbell@citrix.com>