[petsc-users] Bug report: using a custom ksp convergence function

[Barry Smith]

   David,

      KSPConvergedDefault() now takes a non-NULL context that must be provided. I have attached a modified version of ex5f.F90 that demonstrates how it is constructured before being passed to KSPSetConvergenceTest(). 

   Barry
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> On Jan 20, 2025, at 8:54?AM, Daniel Stone <daniel.stone at opengosim.com> wrote:
> 
> Hello PETSc Community,
> 
> I think I've found a bug -
> 
> Go to $PETSC_DIR/src/ksp/ksp/tutorials/
> 
> open ex5f.F90, and add the following line in MyKSPConverged(), say around line 336:
> 
>  call KSPConvergedDefault(ksp,n,rnorm,flag,dummy,ierr)
> 
> It should make sense why someone would want to do this - within a definition of custom convergence behaviour, get the default convergence flags, and, based on certain conditions, overwrite it.
> 
> Now, building and running the exercise, making sure to include the flag to use the custom convergence:
> 
>  boston at boston-SYS-540A-TR:~/DATA_DRIVE/PETSC_DIRS/petsc_3.22/src/ksp/ksp/tutorials$ ./ex2f -my_ksp_convergence
> [0]PETSC ERROR: ------------------------------------------------------------------------
> [0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation, probably memory access out of range[...]
> 
> From my debugging, the issue is at line 1520 of iterativ.c:
> 
>   KSPConvergedDefaultCtx *cctx = (KSPConvergedDefaultCtx *)ctx;
> 
> Because a dummy is used for the context object, this cast fails (my debugger says "cannot access memory..." for cctx). Then the crash happens when trying to access members of cctx:
> 
>   if (cctx->convmaxits && n >= ksp->max_it) {
> 
> 
> I'm not sure how to fix it. Crucially, we must note that that this did work in older versions of PETSc - I can repeat this test in 3.19.1, for example, and it works fine - a debugger shows the cast succeeding and producing some
> defaulted version of the ctx object, the subsequent crash does not happen, etc.
> 
> Does anybody have any advice for working around this for the time being? A piece of software I work with uses a custom ksp convergence test in the manner described above, and is only functional with older versions of petsc because of this.
> Like in ex2f, I have no need of the ctx object, and just as in ex2f, I use 0 instead.
> 
> Things I'll try next: defining a proper ctx object (I use 0, like in ex2f), or looking for some PETSC_NULL_CTX definition somewhere.
> 
> Many Thanks,
> 
> Daniel
> 
> 
>