Hi, I'm using PETSc via the libMesh framework, so creating a MWE is complicated by that, unfortunately.
The situation is that I am not modifying the solution vector in a callback. The SNES solve has terminated, with PetscErrorCode 82, and I then want to update the solution vector (reset it to the "previously converged value") and then try to solve again with a smaller load increment. This is a typical "auto load stepping" strategy in FE. I think the key piece of info I'd like to know is, at what point is the solution vector "unlocked" by the SNES object? Should it be unlocked as soon as the SNES solve has terminated with PetscErrorCode 82? Since it seems to me that it hasn't been unlocked yet (maybe just on a subset of the processes). Should I manually "unlock" the solution vector by calling VecLockWriteSet? Thanks, David On Wed, Dec 17, 2025 at 2:02 PM Stefano Zampini <[email protected]> wrote: > You are not allowed to call VecGetArray on the solution vector of an SNES > object within a user callback, nor to modify its values in any other way. > Put in C++ lingo, the solution vector is a "const" argument > It would be great if you could provide an MWE to help us understand your > problem > > > Il giorno mer 17 dic 2025 alle ore 20:51 David Knezevic via petsc-users < > [email protected]> ha scritto: > >> Hi all, >> >> I have a question about this error: >> >>> Vector 'Vec_0x84000005_0' (argument #2) was locked for read-only access >>> in unknown_function() at unknown file:0 (line numbers only accurate to >>> function begin) >> >> >> I'm encountering this error in an FE solve where there is an error >> encountered during the residual/jacobian assembly, and what we normally do >> in that situation is shrink the load step and continue, starting from the >> "last converged solution". However, in this case I'm running on 32 >> processes, and 5 of the processes report the error above about a "locked >> vector". >> >> We clear the SNES object (via SNESDestroy) before we reset the solution >> to the "last converged solution", and then we make a new SNES object >> subsequently. But it seems to me that somehow the solution vector is still >> marked as "locked" on 5 of the processes when we modify the solution >> vector, which leads to the error above. >> >> I was wondering if someone could advise on what the best way to handle >> this would be? I thought one option could be to add an MPI barrier call >> prior to updating the solution vector to "last converged solution", to make >> sure that the SNES object is destroyed on all procs (and hence the locks >> cleared) before editing the solution vector, but I'm unsure if that would >> make a difference. Any help would be most appreciated! >> >> Thanks, >> David >> > > > -- > Stefano >
